A review of short naps and sleep inertia: do naps of 30 min or less really avoid sleep inertia and slow-wave sleep?

A review of short naps and sleep inertia: do naps of 30 min or less really avoid sleep inertia and slow-wave sleep?

Accepted Manuscript A review of short naps and sleep inertia: Do naps of 30 min or less really avoid sleep inertia and slow wave sleep? Cassie J. Hild...

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Accepted Manuscript A review of short naps and sleep inertia: Do naps of 30 min or less really avoid sleep inertia and slow wave sleep? Cassie J. Hilditch, PhD, Jillian Dorrian, PhD, Siobhan Banks, PhD PII:

S1389-9457(17)30002-3

DOI:

10.1016/j.sleep.2016.12.016

Reference:

SLEEP 3268

To appear in:

Sleep Medicine

Received Date: 31 October 2016 Accepted Date: 28 December 2016

Please cite this article as: Hilditch CJ, Dorrian J, Banks S, A review of short naps and sleep inertia: Do naps of 30 min or less really avoid sleep inertia and slow wave sleep?, Sleep Medicine (2017), doi: 10.1016/j.sleep.2016.12.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Title A review of short naps and sleep inertia: Do naps of 30 min or less really avoid sleep inertia

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and slow wave sleep?

Authors & Affiliations

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Cassie J Hilditch, PhD1,2 ; Jillian Dorrian, PhD2 ; Siobhan Banks, PhD2 E.P. Bradley Sleep Lab, Brown University, Providence, RI, USA

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Centre for Sleep Research, University of South Australia, Adelaide, SA, Australia

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Correspondence

Cassie J Hilditch, Centre for Sleep Research, University of South Australia, GPO Box 2471, Adelaide, SA, 5001, Australia. Tel: +61 8 8302 2894; Fax: +61 8 8302 6623; email:

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[email protected]

Conflicts of Interest

Please see attached COI forms.

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ACCEPTED MANUSCRIPT Abstract Objectives Napping is a widely used countermeasure to sleepiness and impaired performance caused by sleep loss and circadian pressure. Sleep inertia, the period of

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grogginess and impaired performance experienced after waking, is a potential side-effect of napping. Many industry publications recommend naps of 30 minutes or less in order to avoid this side-effect. However, the evidence-base for this advice has yet to be thoroughly

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reviewed.

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Methods Electronic databases were searched and defined criteria applied to select articles for review. The review covers literature on naps of 30 min or less in relation to: a) sleep inertia, b) slow wave sleep (SWS), and c) the relationship between sleep inertia and SWS. Results The review found that while the literature on short afternoon naps is relatively

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comprehensive, there is very little literature on naps of 30 minutes or less at night. Studies have mixed results as to the onset of SWS, and the duration and severity of sleep inertia following short naps, making guidelines regarding their use unclear. The varying results are

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waking.

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likely due to differing sleep/wake profiles prior to the nap of interest and the time of day at

Conclusions The review highlights the need to have more detailed guidelines about the implementation of short naps based on time of day and prior sleep/wake history. Without this context, such a recommendation is potentially misleading. Further research is required to better understand the interactions between these factors, especially at night, and to provide more specific recommendations.

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ACCEPTED MANUSCRIPT Key Words Alertness; Best practice; Evidence-based practice; Performance; Napping policy; Shift work;

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Sleepiness; SWS

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ACCEPTED MANUSCRIPT Introduction Napping is a widely used countermeasure to sleepiness and impaired performance caused by sleep loss, extended wakefulness, or being awake during the night (1-6). Sleep inertia,

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the period of grogginess and impaired performance experience after waking, is often cited as a negative side-effect of napping (2, 6-11). Indeed, the effects of sleep inertia can be even more extreme than a night of sleep loss (12). This period of impairment has been officially

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implicated in several real-world accidents and incidents (13-16).

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Sleep inertia has been observed after waking from a full, habitual night of sleep under wellrested conditions, and thus appears to be a ubiquitous step in the sleep-wake transition (12, 17, 18). There are, however, several key contributors to sleep inertia which have emerged from the literature. Conditions that tend to exacerbate sleep inertia include: waking during the circadian low in alertness (biological night) (19-21);

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prior sleep loss, or extended wakefulness prior to a sleep episode (19, 22); and

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sleep depth (slow wave sleep, SWS, in the prior sleep period; or waking from deeper

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-

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sleep stages) (19, 23, 24).

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Napping is a recommended sleepiness countermeasure in industry-focused journal articles (10, 25-28) and industry best practice and government work safety publications (29-31). The guidelines for implementation of this countermeasure, however, are often vague. Typically, a general statement is made about the potential for impaired performance after waking, but rarely are any specific details provided as to the severity or duration of sleep inertia following different napping situations. What is often cited, however, is that naps of 30 minutes or less (short naps) are generally considered to be effective to avoid sleep inertia

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ACCEPTED MANUSCRIPT (32). The National Sleep Foundation even recommends a short nap to avoid grogginess but does not mention the potential influence of the time of day (33). Thus, it is broadly assumed that these nap lengths are “safe”, with respect to the risk of sleep inertia. The recommendation to take a short nap to avoid sleep inertia, however, is based on studies

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that have largely been conducted during the day, under conditions of limited wakefulness (9, 11, 34). How these conditions translate to different scenarios common in the workplace

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(e.g. naps on night shift) has not been comprehensively reviewed.

This leads to the overarching question: what is the evidence for recommending short naps

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to avoid sleep inertia? This review aims to address this question by investigating the three main assumptions of this recommendation as they relate to short naps: (a) the absence of sleep inertia; (b) the absence of SWS; and (c) the relationship between SWS and sleep

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inertia.

To investigate these assumptions, three structured narrative reviews (i.e. narrative reviews of articles identified through a systematic search) were conducted. A structured narrative

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review approach was chosen to allow the comparison of findings from a broad range of literature including both laboratory and field studies (35). The first assumption (absence of

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sleep inertia) was addressed by a systematic search to identify articles that investigated performance and/or alertness in the first 30 minutes after waking from short naps. The articles identified by these searches are summarised in Table 1 (daytime naps) and Table 2 (night-time naps). In order to address the second assumption (absence of SWS), articles identified in the first search were screened for SWS data, and an additional search for articles containing data on SWS in short naps (without sleep inertia data) was conducted. The articles identified by these searches are summarised in Table 3. To address the third

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ACCEPTED MANUSCRIPT assumption (relationship between SWS and sleep inertia), articles that met the combined inclusion criteria for the previous two searches were reviewed. These articles are identified

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in Tables 1 and 2 by an asterisk in the Weight of Evidence (WOE) classification column.

Assumption 1: Sleep inertia and short naps

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Search methods

Three electronic databases (PubMed, Science Direct and Scopus) were searched on

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24 July 2015 using the combined terms “sleep inertia” AND “nap”, resulting in 115 articles. After filtering for duplicates (N=51) and studies not meeting the inclusion criteria outlined below (N=53), 11 articles remained. An additional nine articles were manually retrieved from reference lists. The search was run again on 27 June 2016 to capture recently

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published literature. This search identified a further two relevant articles (21, 36). A total of 22 articles met the inclusion criteria (Figure 1). These articles have been summarised in two

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tables: Table 1 contains studies investigating daytime naps (N=17) (naps taken between 07:00–23:00) (37); while Table 2 contains studies investigating night-time naps (N=8) (naps

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taken between 23:00–07:00). Three studies were included in both tables as they investigated both daytime and night-time naps (6, 38, 39). Articles within each table are ordered by nap length. Where a study included several nap lengths, the article appears in line with the shortest nap. The narrative review is also structured by nap timing and nap length. Studies in Tables 1 and 2 met the following inclusion criteria: original article (e.g. not a review); published in a peer-reviewed journal; written in English; used healthy adult human

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ACCEPTED MANUSCRIPT participants; investigated a nap of 30 minutes or less; tested alertness and/or performance within or at 30 minutes after nap end; and compared results to a no-nap control group. Articles were further classified into higher WOE (N=12) or lower WOE (N=10) articles. Higher WOE articles met stricter inclusion criteria that allowed for a more definitive interpretation

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of the results. The additional inclusion criteria were: a testing point within 30 minutes after nap end; at least two testing points within 45 minutes of nap end; and distinct test bouts

stricter criteria but met the overall inclusion criteria.

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(i.e. no data binning greater than 15 minutes). Lower WOE articles did not meet these

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All studies in Tables 1 and 2 were experimental: 18 were laboratory-based while four were field-based (i.e. at shift workers’ places of work). Outcome measures, testing points, timing of awakening and length of prior sleep varied, thus direct comparisons between studies was

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difficult, making a narrative analysis of results the most viable option.

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Online databases

(N=117)

Reference lists

(N=9)

TOTAL

(N=126) Duplicate records excluded (N=51)

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Articles screened

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Articles retrieved from searches

(N=75)

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Articles excluded

(N=1)

Not original research

(N=22)

Did not use healthy adult human participants

(N=6)

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Not in English

(N=20)

First test point >30 min

(N=2)

No control group

(N=2)

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Nap >30 min

TOTAL

(N=53)

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Articles included in tables (N=22)

Figure 1 Flow chart illustrating the systematic selection process for articles populating Tables 1 and 2 and discussed in the structured narrative review

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Sleep inertia and short naps: Daytime

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This section explores what is known about performance and alertness immediately following short daytime naps ranging from 30 seconds to 30 minutes in duration, taken outside of typical sleeping hours (i.e. between 07:00 and 23:00) (37). Studies are organised

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with regard to the nap length. One study investigated a range of nap lengths and is, therefore, referred to repeatedly throughout this section (11). This repeated measures

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study involved five napping conditions (0, 5, 10, 20 or 30-min nap) ending at 15:00 (11). Nap length was measured from sleep onset, rather than being a fixed TIB opportunity (i.e. nap conditions were defined by TST). Participants were sleep restricted to five hours TIB the

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≤5-min naps

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previous night.

No effects were observed on post-nap performance or alertness in a study of ultra-brief

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naps (30 and 90 seconds) (34). Similarly, studies have found no difference in immediate performance following 3-5-min naps (11, 38, 40, 41). With regard to alertness, one study showed an improvement in subjective alertness within 15 minutes of waking (41). Conversely, Bastuji et al. (40) noted a decrease in a measure of cognitive arousal, although other measures remained stable and performance was unaffected. From these studies, it would appear that sleep inertia does not manifest as objective performance impairment or

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ACCEPTED MANUSCRIPT subjective sleepiness following a nap of up to five minutes in the daytime. These findings also indicate a lack of immediate performance benefits from naps of this length.

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10-min naps

Brooks and Lack (11) found improvements following a 10-min nap, compared to the no-nap

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condition, for all performance measures and subjective scales at almost all half-hourly testing time points from 5–155 minutes’ post-nap. This finding suggests that the benefits of

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a 10-min nap in the afternoon are immediate and relatively long-lasting, with no sleep inertia. While this study replicated the findings of a previous study (9), it did not support the findings of another study by the same authors (34). In the latter study (34), a 10-min nap showed no significant improvement in alertness or performance at the first test point five

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minutes after waking. The 10-min nap, however, did confer improvements on some measures at 35 minutes. While there was no significant deterioration of performance five minutes post-nap, it is possible that mild sleep inertia delayed or masked the benefits of the

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10-min nap in the 2002 study, as significant improvements were first seen at 35 minutes.

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The sleep architecture of the 10-min naps was not discussed in these papers, so it is unknown whether there were any differences between the two 10-min nap conditions and whether this may have influenced the results. The difference cannot be explained by prior sleep/wake (5 hours TIB; 10 hours of wakefulness) or time-of-day (15:00) conditions, as these were identical between studies (9, 34). In summary, while no study of 10-min afternoon naps has reported sleep inertia per se, not all studies have reported immediate improvements to performance and alertness.

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ACCEPTED MANUSCRIPT However, a 10-min nap taken in the morning (07:00) following a simulated night shift (i.e. under conditions of prior extended wakefulness) was associated with sleep inertia (36). In this study, significantly worse performance was observed immediately after waking compared to pre-nap and a no-nap control group. No difference was found between groups

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on a simulated driving task undertaken from 5–45 minutes after waking. It is unknown whether sleep inertia was short-lived, or whether the driving task was not sensitive enough

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to detect longer lasting sleep inertia.

Therefore, while studies of 10-min naps following moderate sleep restriction show

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promising results for immediate performance benefits, a 10-min nap following prior extended wakefulness does not appear to confer the same homeostatic benefits and shows

15-min naps

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signs of sleep inertia.

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While several studies of 15-min naps during the day have been conducted (42-48), few have systematically investigated the potential for sleep inertia following these naps. Of the

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studies meeting our inclusion criteria, only one was classified as a higher WOE article (46). Kaida et al. (46) measured performance and alertness every five minutes across the first 30 minutes after waking. Performance was significantly improved 15 minutes after waking, but was not different to the no-nap condition at earlier testing points. This delay may have been due to short-acting sleep inertia masking the initial performance benefits. No changes were observed in subjective sleepiness. Takahashi and colleagues investigated the effect of a 15min afternoon nap opportunity on cognitive arousal levels, self-reported sleepiness and

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ACCEPTED MANUSCRIPT objective performance on a logical reasoning task (44, 45). The first testing point was 30 minutes after the nap, thus potentially missing the critical inertia period. At 30 minutes post-nap, the authors found that cognitive response latencies were shortened, but no change was observed in response amplitude compared to pre-nap. Subjective sleepiness

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was also reduced and there were modest improvements in performance. Another paper from this group reported the effects of a lunch-time 15-min nap taken daily for a week at a

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workplace (48). Again, the first testing point was 30 minutes after waking and, in this case, only included subjective alertness ratings. Towards the end of the week, alertness was

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significantly improved relative to the no-nap condition. Taken together, these studies of 15min afternoon naps suggest that low levels of sleep inertia may exist up to 15 minutes after

20-min naps

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waking, but that nap benefits may be observed within 15–30 minutes of waking.

Brooks and Lack’s (11) investigation of a 20-min afternoon nap found that performance

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improvements were not observed until at least the second testing point at 35 minutes post-

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nap. Furthermore, there were negative trends for subjective sleepiness and objective performance at five minutes post-nap relative to pre-nap. As in Kaida et al.’s (46) findings, this suggests that a degree of sleep inertia occurred following the 20-min nap which masked the benefits immediately after the nap. Hayashi and colleagues have conducted several 20-min afternoon napping studies (49-54). The earlier studies from this group first tested participants approximately 20 minutes after the nap (49-51). However, data from this first time point were combined with test points

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ACCEPTED MANUSCRIPT across the following hour in analyses, which may have obscured sleep inertia across this period. These earlier studies were therefore excluded from this review. In Hayashi et al.’s (52) study of countermeasures to sleep inertia, testing began two minutes after waking and was systematically repeated across the first hour. In this study, no differences were

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observed within 15 minutes of waking for performance or for subjective and objective alertness between the nap group and no-nap group. Sleepiness ratings reduced after 15

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minutes in the nap condition, but this was not reflected in objective performance measures. Another study by Hayashi et al. (54) also found that subjective alertness and performance

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did not differ between nap and no-nap groups in the first 10 minutes after waking. In contrast, another study from this group reported an increase in sleepiness scores immediately after waking from a post-lunch nap (53). The next testing point was over an hour later, making the time course of sleep inertia difficult to determine.

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Under conditions of sleep deprivation, studies of 20-min daytime naps report that sleep inertia is evident immediately after waking, especially on measures of reaction time (6, 39).

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Performance then returned to, but did not improve upon, pre-nap levels 15 minutes after waking (6). These studies highlight that, under conditions of sleep pressure, a 20-min

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daytime nap is not short enough to avoid sleep inertia and any homeostatic benefits of the nap may be outweighed by the sleep inertia. Under well-rested conditions, 20-min naps appear to provide benefits but these are delayed by approximately 15 minutes.

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ACCEPTED MANUSCRIPT 30-min naps Studies of 30-min naps during the day have consistently found evidence of brief sleep inertia. Two similar studies investigated the costs and benefits of a 30-min nap taken in the

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afternoon following a night of sleep restriction (5-h TIB opportunity) (9, 11). At five minutes post-nap, performance was significantly lower compared to pre-nap levels (9) and worse than subsequent testing points (11). For both studies, at 35 minutes post-nap, no

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differences were observed for most performance measures compared to pre-nap and the no-nap condition. At 65 minutes post-nap, however, both subjective and objective

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sleepiness were reduced compared to the no-nap condition (9, 11). These results suggest that sleep inertia was evident for at least five minutes after a 30-min afternoon nap and that benefits of the nap appeared after 60 minutes. Given the time between testing points, it is unknown how long the detriments due to sleep inertia lasted, and when benefits emerged.

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The return to pre-nap levels at 35 minutes suggests that, while there was no significant decrease in performance, sleep inertia effects may still have masked nap benefits, leading to

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a delay in observed benefits.

Gillberg et al. (55) tested whether a 30-min nap in the morning following sleep restriction

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(4-h TIB opportunity) could bring neurobehavioural outcomes back to levels observed after a full night’s sleep (baseline; 7.5-h TIB opportunity). Their first testing point was at 30 minutes post-nap. At this time, performance on a 28-min vigilance task, objective sleepiness (as measured by EEG) and subjective sleepiness ratings were all not significantly different to baseline levels. This study, although having deliberately delayed testing for the first 30 minutes after waking in order to avoid sleep inertia effects, was in line with previous studies

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ACCEPTED MANUSCRIPT which show no difference relative to pre-nap or the no-nap condition at approximately 30 minutes after the nap.

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Summary of short daytime naps

From this review of short daytime naps, it appears that sleep inertia is immediately evident

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after a 30-min nap with significantly worse performance relative to pre-nap and no-nap levels. Following a 15–20-min nap, there is likely to be no change to performance and

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alertness in the first 15 minutes after a nap, with eventual benefits initially masked by mild sleep inertia. Ten-minute naps in the afternoon are often associated with immediate benefits, while naps of five minutes or less tend to have no effect on performance, either positive or negative. However, a 10-min nap taken in the morning following a night of sleep

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loss resulted in sleep inertia. Thus, naps of 10–20-min in the afternoon under well-rested or moderate sleep restriction conditions appear to have the best cost/benefit trade-off. Under conditions of greater sleep loss, however, 10–20-min daytime naps exhibit significant sleep

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inertia. Therefore, prior sleep–wake history should always be taken into account when

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estimating sleep inertia effects, even when taking short naps in the daytime.

Sleep inertia and short naps: Night-time Very few studies have been conducted on naps of less than 30 minutes taken at night, and fewer still have investigated sleep inertia. This area of investigation is important, as shorter nap lengths are potentially more operationally viable on night shifts (i.e. short naps require less time away from work). Given that a 10-min nap has been shown to provide immediate

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ACCEPTED MANUSCRIPT and sustained performance benefits in the afternoon (9, 11), the potential benefits of this nap length at night warrant attention. This section summarises the available research on

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short night-time naps.

10-min naps

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Hilditch et al. (21) measured performance and sleepiness at 2, 17, 32 and 47 minutes after a 10-min nap ending at 04:00 during a simulated night shift. In contrast to the benefits

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observed following 10-min naps in the afternoon, no change in performance or sleepiness was observed from pre- to post-nap. These findings suggest that near the circadian nadir, following extended wakefulness, a 10-min nap is not able to improve performance. The nap did, however, stabilise performance relative to the decline observed in the no-nap

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20-min naps

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condition.

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Signal et al. (6) compared a 20-min nap to no-nap, or a 40-min or 60-min nap ending at 02:00 following 20 hours of wakefulness. The sleep inertia period was systematically measured with frequent test bouts, beginning immediately upon waking and repeated every 15 minutes for an hour. No change was observed in subjective sleepiness pre- to post-nap for any condition. In contrast to the 40-min and 60-min conditions, performance during the first test bout following the 20-min nap was not significantly worse than pre-nap, suggesting a lack of sleep inertia. Visual inspection of the graphs revealed a trend towards worse performance immediately after waking in the 20-min condition. Under conditions of greater

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ACCEPTED MANUSCRIPT sleep deprivation, Naitoh et al. (39) observed significant sleep inertia immediately following 20-min naps taken every six hours. Sleep inertia effects in this study were so intolerable that 40% of participants in the napping group withdrew from the study, compared to no

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withdrawals in the control (64 hours’ sleep deprivation) condition. In the field, Purnell et al. (56) studied aircraft maintenance engineers across a double night shift in a cross-over design featuring a 20-min nap condition and a no-nap rest break

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condition. The nap was taken at any time between 01:00–03:00 according to operational demands (naps tended to be closer to 03:00). In all, 50% of participants reported that they

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were unable to sleep during the nap opportunity. Performance and subjective fatigue were compared pre-nap to a single test point 30 minutes post-nap. Analysis revealed that subjective fatigue and response times on a 5-min vigilance task were significantly worse post-nap compared to pre-nap. Response time and the number of missed responses on a 2-

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min simple reaction time task were not affected. Tests were not performed at equivalent times during the no-nap condition, however, so the change in performance may have been

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due to circadian effects. A similar study design with time-matched test points before and after a non-sleep rest break is needed to clarify this observation.

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With only these few studies of 20-min night-time naps to compare, it is unclear as to whether sleep inertia can be completely avoided or whether there are any benefits from these naps.

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ACCEPTED MANUSCRIPT 30-min naps Many studies of 30-min naps at night lack a systematic assessment of performance and alertness within 30 minutes of waking (38, 57-59). To address this, Hilditch et al. (21)

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studied a 30-min nap ending at 04:00 during a simulated night shift. As described above, performance and fatigue was measured at 15-min intervals for the first hour after waking. The study showed significant performance impairment immediately after waking, with

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performance still worse than pre-nap at 47 minutes after waking. Interestingly, self-rated performance did not match objectively measured performance with participants reporting

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improved performance post-nap. These findings suggest that significant, long-lasting sleep inertia is present following a 30-min nap near the nadir under extended wakefulness conditions. It also highlights the potential for people to over-estimate the benefits of a nap under these conditions and the subsequent need for both subjective and objective

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measures when assessing sleep inertia.

Other studies first tested performance and subjective sleepiness approximately 10–15

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minutes after participants had woken from a night-time 30-min nap (57, 58). Both studies found a significant reduction in subjective sleepiness for the nap group at the first testing

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point, compared to those in the no-nap group. However, no differences were observed in objective performance on a range of tasks. Neurobehavioural outcomes were not measured again until at least 30 minutes later, at which point no differences were found between the nap and no-nap groups. Significant benefits of the naps were not observed until at least one hour after waking. Given the lack of frequent testing points post-nap, the time course of sleep inertia is not clear from these studies. In addition, given the first objective measure

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ACCEPTED MANUSCRIPT was taken 10–15 minutes post-nap, this may explain the lack of observed performance impairment if sleep inertia was short-lived. Indeed, Sallinen et al. (59) reported significantly worse performance immediately after

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waking from a 30-min night-time nap compared to 15 minutes later. Subjective sleepiness was not reported during this period. Unfortunately, there were no further testing points in this study, nor was there a pre-nap background test point against which to compare the 15-

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min post-nap test point. Therefore, the time course of sleep inertia, and whether performance had returned to background levels at 15 minutes post-nap, could not be

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determined. Another 30-min night-time study tested performance immediately after waking and found worse performance post-nap compared to pre-nap (38). However, this change was also observed in the no-nap group, and there were no subsequent testing points to

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determine whether the effect was circadian-driven or due to sleep inertia. Together, these studies suggest that there is a degree of sleep inertia associated with 30min naps at night, and that performance and alertness may take at least 45 minutes to

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return to pre-nap levels. Benefits of 30-min naps were not observed within an hour of waking. Comprehensive testing during the first hour (as per (21)) is needed to provide a

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clearer picture of the profile of sleep inertia following 30-min naps at night.

Summary of short night-time naps Studies of naps at night are often associated with sleep deprivation and extended wakefulness, which confounds any potential direct circadian effects. In the findings from the limited number of studies available to review, a 10-min night-time nap appears to stabilise

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ACCEPTED MANUSCRIPT performance but does not provide the immediate benefits observed in the afternoon. Longer night-time naps (20-min and 30-min) appear to behave similarly to daytime naps, especially daytime naps under conditions of sleep restriction. Sleep inertia is observed immediately after waking from 30-min night-time naps, with recovery to pre-nap levels seen

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after 30–60 minutes, and significant benefits after 60 minutes. Results were mixed following 20-min naps, with some studies reporting significant impairment, but others showing no

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change. Only one study has been conducted on sleep inertia following naps shorter than 20 minutes at night. This research area requires further research to confirm existing findings.

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Conclusions regarding the effect of time of day are difficult to draw, given the scarcity of night-time studies with a systematic investigation of sleep inertia and no direct comparisons of short daytime and night-time naps. More frequent testing during the first hour after waking would help to better identify the profile of sleep inertia following short night-time

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naps. Specifically, the half-hour window from 5–35 minutes post-nap has been underinvestigated. From an operational point of view, understanding performance levels across

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this period is important for both safety (leaving enough time for recovery) and productivity

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(returning to work as soon as possible).

Assumption 1: Summary

Table 4 summarises the immediate performance effects following short naps of increasing length. Naps of 10–20 minutes appear to be beneficial for immediate performance when taken during the daytime under well-rested conditions. Greater prior sleep loss before a nap increases the risk of sleep inertia, with the result that, even during the daytime,

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ACCEPTED MANUSCRIPT performance following 10–20-min naps may exhibit signs of sleep inertia. For example, while 10-min naps are recommended for a daytime performance boost, this nap length does not appear to reap the same benefits under night shift conditions following extended wakefulness and under greater time-of-day pressures. Therefore, it is important that

similar conditions.

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Assumption 2: Sleep inertia and slow wave sleep (SWS)

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napping advice for night shift workers is based on empirical evidence of naps taken under

Search methods

The articles shown in Tables 1 and 2 were searched to identify those that met the new inclusion criteria for the investigation of Assumption 2. This search identified 16 articles. In

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order to capture studies not identified in the first search, three electronic databases (PubMed, Science Direct and Scopus) were searched on 18 August 2015 using the combined

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terms “slow wave sleep” AND “nap”, resulting in 139 articles. After filtering for duplicates (N=60) and studies not meeting the inclusion criteria outlined below (N=73), six articles

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remained. An additional three articles were manually retrieved from reference lists. Repeating the search on 27 June 2016 did not return any recent additional articles other than the two already identified in the search for Assumption 1. Figure 2 displays this selection process, and Table 3 is comprised of the 25 articles meeting the inclusion criteria. Three articles were combined in a single row as they all report on studies of the same participants (4, 6, 60). Articles in Table 3, and the narrative review below, are structured according to prior sleep/wake conditions.

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ACCEPTED MANUSCRIPT Studies in Table 3 met the following inclusion criteria: original article (e.g. not a review); published in a peer-reviewed journal; written in English; used healthy human adult participants; investigated a nap of 30 minutes or less; measured sleep using polysomnography; and reported sleep architecture details as scored using Rechtschaffen

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and Kales (61) or AASM (62) rules. All but one study in Table 3 was experimental: the exception was an observational design collating data across several studies. The majority of

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studies were laboratory-based, two studies were field-based.

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ACCEPTED MANUSCRIPT

Online databases

(N=141)

Reference lists and previous search

(N=17)

TOTAL

(N=158) Duplicate records excluded (N=60)

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Articles screened

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Articles retrieved from searches

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(N=98) Articles excluded

(N=2)

Not original research

(N=7)

Did not use healthy adult human participants

(N=12)

Nap >30 minutes

(N=43)

SWS not reported

(N=3)

Not relevant

(N=6)

TOTAL

(N=73)

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EP

TE D

Not in English

Articles included in table (N=25)

Figure 2 Flow chart illustrating the systematic selection process for articles populating Table 3 and discussed in the structured narrative review

23

ACCEPTED MANUSCRIPT

Sleep architecture of short naps

RI PT

Under well-rested conditions, SWS typically begins approximately 30 minutes after sleep onset in the first cycle of a habitual nocturnal sleep bout (63). Naps of 30 minutes or less, therefore, have been posited to avoid SWS, at least under the same conditions. However,

SC

SWS is homeostatically controlled, with SWS onset advancing, and its duration lengthening, under conditions of prior sleep loss, extended wakefulness or prior SWS restriction (19, 64-

M AN U

68). There is no known independent time-of-day effect on SWS (69, 70).

The focus of this section is on SWS onset and duration as assessed in short naps (≤30 minutes). Of the studies identified and presented in Table 3, five were conducted

TE D

under conditions of extended wakefulness, and a further four were conducted under sleep restriction conditions with limited prior wakefulness. All but one of the remaining studies targeted afternoon napping following a habitual nocturnal sleep period. The exception was

EP

a study that investigated a night-time nap preceded by a full nocturnal sleep period and a 2-

AC C

h afternoon nap, such that prior wakefulness was truncated (57). In addition, Howard et al.’s (38) study included two napping conditions: one under extended wakefulness (night-time nap), the other under well-rested conditions (evening nap). Unless specified otherwise, all references to nap length in this section describe the total sleep time of the nap (i.e. TST), not the length of the nap opportunity (i.e. TIB).

24

ACCEPTED MANUSCRIPT Well-rested conditions Of the 13 studies investigating short afternoon or evening naps under well-rested conditions, eight reported no SWS in any participants. Two studies reported two of

RI PT

10 participants entering SWS for an average of 1.5–4 minutes within 15–20 minutes of sleep (50, 54). Another study reported an average 6.6 minutes of SWS in a 30-min nap (71). Fushimi and Hayashi (72) pooled data from several studies and found that SWS was only

SC

entered in naps longer than 15 minutes, with an average of up to five minutes of SWS in naps of 20–30 minutes. On average, SWS appeared 18 minutes after the onset of Stage 1

M AN U

sleep (72). In summary, it appears that under well-rested conditions, an afternoon nap of less than 15 minutes is unlikely to contain SWS; however, naps of 15–30 minutes may be

Restricted sleep

TE D

comprised of up to 23% SWS.

EP

Three studies investigated short afternoon naps following restricted nocturnal sleep (4–5-h TIB opportunity) and found no SWS in naps of less than 10 minutes (11, 41). However, some

AC C

SWS occurred in naps of 10 minutes or more (11, 55), with a 30-min nap containing over 30% SWS (11). Under more severe sleep restriction (75 minutes TST in the prior nocturnal sleep period), a 30-min nap opportunity produced an average of 23 minutes TST, 15 minutes of which was SWS (73). Therefore, afternoon naps following moderate sleep restriction are unlikely to contain SWS if they are shorter than 10 minutes, but are likely to contain SWS if they are longer than 15 minutes. The severity of prior sleep restriction increases the likelihood of early SWS onset.

25

ACCEPTED MANUSCRIPT

Extended wakefulness Under conditions of extended prior wakefulness (~20 hours), night-time naps as short as 8

RI PT

minutes TST contained, on average, 0.8 minutes of SWS (21). Longer naps with 15– 20 minutes TST contain a broad range of SWS length (0.7–7 minutes) (4, 6, 39, 58). Similarly, of the three napping studies with 20–27 minutes’ sleep, the average length of SWS ranged

SC

from 4.4–14.7 minutes (21, 38, 59). Paradoxically, a 30-min night-time nap following full rest

M AN U

and a 2-h afternoon nap resulted in 17.8 minutes of SWS (57), more than similar naps taken following uninterrupted wakefulness (21, 59). While moderate levels of wakefulness extension (~20 hours) appear to modestly advance SWS onset, under greater wakefulness extensions of 24–64 hours, the amount of SWS in naps of similar length significantly

TE D

increases (4, 6, 36, 39).

As with afternoon naps following sleep restriction, the onset latency of SWS is likely to be between 10 and 20 minutes for naps at night under a moderate wakefulness extension.

EP

However, under these conditions, SWS onset has been observed within 10 minutes of sleep

AC C

onset. Increasing prior hours of wakefulness further advances SWS onset, such that SWS onset is likely within a 10-min nap.

Slow wave sleep (SWS) metrics and individual differences When interpreting studies which report the mean duration of SWS in a short nap, the range of SWS onset across participants and the proportion of participants actually entering SWS during a nap both present challenges. The average values of SWS duration often contain

26

ACCEPTED MANUSCRIPT many zeros (i.e. participants not entering SWS); therefore, the data are not normally distributed. Furthermore, the low number of participants in some studies makes it difficult to generalise findings to a broader population. The study by Smith et al. (58) illustrates this point. The authors reported individual sleep data from six participants in a field study of a

RI PT

30-min nap opportunity. Of these six participants, three entered SWS with durations ranging from 4.4–10.5 minutes. The mean SWS duration was 3.3 minutes for this group. This mean,

SC

however, does not necessarily give an indication of the likelihood of entering SWS, or of the average SWS duration for those participants who entered SWS. Similarly, Hayashi et al. (50)

M AN U

reported individual participant results with two of the 10 participants entering SWS for three and five minutes, respectively, during a 20-min nap. The average SWS duration was 0.8 minutes for the group, but this value does not necessarily meaningfully reflect the actual value for any participant. The proportion of participants entering SWS at each nap length

EP

Assumption 2: Summary

TE D

may provide a better indicator of the likelihood of SWS in a short nap.

AC C

Table 4 summarises the presence of SWS in short naps with increasing nap length. Studies reporting the proportion of participants entering SWS in naps of up to 30 minutes show that this proportion increases linearly with increasing nap duration (11, 72). This has been observed under both well-rested (72) and sleep-restricted conditions (11). The principle that shorter naps reduce the chance of entering SWS is supported by these findings. However, SWS has been shown to occur within 15–20 minutes of sleep onset in well-rested conditions, and within 10–15 minutes in naps following sleep restriction or extended wakefulness. Therefore, the assumption that a short nap (≤30 minutes) always avoids SWS

27

ACCEPTED MANUSCRIPT is unsupported. Although the estimates of nap lengths associated with SWS onset appear to match the observations of sleep inertia discussed in the previous section, this finding does not, ipso facto, mean that short naps with SWS are associated with sleep inertia. The evidence for a direct relationship between SWS and sleep inertia in short naps is discussed

RI PT

in the next section.

The findings from the current section suggest that SWS may occur in naps as short as 10–

SC

15 minutes. This is especially the case under the condition of extended wakefulness, a condition common to night shift naps. Therefore, it is necessary to measure sleep with EEG

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EP

TE D

conditions of extended wakefulness.

M AN U

in order to determine the presence of SWS when investigating short naps, especially under

28

ACCEPTED MANUSCRIPT Table 4 Summary of sleep inertia and SWS associated with short naps by length and timing of nap Nap duration (min)

Daytime

Night-time

Contains

Immediate

Contains

Performance

SWS

Performance

SWS

≤5

=

No

?

?

6-10



No

=/↓

Yes

11-15

=

No/Yes*

16-20

=/↓

Yes

21-30



SC

RI PT

Immediate

?

=/↓

Yes

M AN U

?

Yes



Yes

Notes: *Yes, if under conditions of prior sleep loss. Upwards arrows represent improved performance; downwards arrows represent impaired performance; equal signs represent no

AC C

EP

TE D

change in performance relative to pre-nap; ? = unknown.

29

ACCEPTED MANUSCRIPT Assumption 3: SWS and sleep inertia in short naps Search methods Studies meeting the inclusion criteria for higher WOE articles in the first search were cross-

RI PT

referenced with studies that met the inclusion criteria for the second search. This resulted in nine articles that were included in the structured narrative review below. These articles are

SC

identified in Tables 1 and 2 by an asterisk in the WOE classification column.

M AN U

Evidence for the relationship between SWS and sleep inertia in short naps Sleep periods >30 minutes

The proportion of SWS in a sleep period, or waking from a deeper sleep stage, has been

TE D

associated with impairments in performance and alertness upon waking following long naps and nocturnal sleep periods (19, 23, 24, 74). Indeed, a review of sleep inertia concluded that it is “absolutely clear” that waking from SWS leads to greater sleep inertia (7). An

EP

examination of the references used to support this claim revealed a paper which claimed no

AC C

relationship between sleep stage and sleep inertia (75), a published military report which did not measure EEG (76) and a paper that did not measure sleep inertia (77). More recent studies have not found an association between sleep depth and sleep inertia (17, 18, 20, 7880). It should be noted, however, that some of these more recent studies were limited by methodological constraints such as low numbers. For example, the statistical power in some studies reporting no sleep stage effects may have been too low to detect differences, or some studies may not have included SWS in analyses due to too few of their participants waking from this stage (17, 20, 78-80). While the evidence supporting a direct association

30

ACCEPTED MANUSCRIPT between sleep depth and sleep inertia in longer sleep episodes may be contested, the evidence for such a relationship in short naps has not yet been reviewed.

RI PT

Sleep periods ≤30 minutes

Only two studies identified in the search statistically evaluated the influence of sleep stages

SC

in a short nap on subsequent performance and alertness (6, 21). Signal et al. (6) examined 20-, 40- and 60-min naps ending at 02:00 following 20 hours of wakefulness, and at 12:00

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following 30 hours of wakefulness. Sleep stages were categorised into three groups: Stage 1 and Stage 2; SWS; and wake. Performance was more impaired immediately after waking from any sleep stage compared to already being awake. When the nap ended at 12:00, waking from SWS was also significantly worse than already being awake. This may have

TE D

been due to the greater sleep pressure following the nap ending at 12:00 compared to after the night-time nap. The authors also assessed the relationship between the amount of SWS in the nap and subsequent performance but did not find any significant associations. In

EP

contrast, the influence of nap duration was significant, with worse performance observed

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immediately after waking from the longer naps (i.e. 40- and 60-min naps). The amount of SWS significantly increased with nap duration for both the 12:00 and night-time naps. Therefore, it appears that nap duration was more influential than the amount of SWS per se, for sleep inertia.

Hilditch et al. (21) found a significant positive correlation between the magnitude of sleep inertia and both TST and SWS in a 30-min nap opportunity. There were no significant correlations for a 10-min nap opportunity. Therefore, an effect of SWS on sleep inertia

31

ACCEPTED MANUSCRIPT independent of sleep duration was not observed. The study was limited by a small sample size, so these findings should be interpreted with caution. Unexpectedly, studies reporting no or minimal SWS within a nap do not report immediate

RI PT

performance benefits, but rather see no change from no-nap or pre-nap levels for the first 10–15 minutes of testing (41, 46, 52, 54). Conversely, a 30-min nap comprised of over 50% SWS, and from which 91% of participants woke from SWS, did not result in any performance

SC

impairment following the nap (57). However, in this study, the first testing point was 15 minutes after waking. Furthermore, Brooks and Lack (11) investigated sleep architecture in

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naps ranging from 5–30 minutes and found that the occurrence of delta waves (slow waves) was associated with performance improvement following the nap, not performance impairment. Therefore, these studies suggest that the association between SWS and

sleep episodes.

EP

Assumption 3: Summary

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performance after waking from short naps is not consistent with earlier studies on longer

AC C

Together, these results do not support a direct association between SWS and sleep inertia in short naps. Rather, they suggest that SWS, for naps of this length, may be too co-dependent on nap duration to independently influence subsequent performance. Therefore, even with no SWS in a nap, there is still the potential for sleep inertia. This highlights the need for studies measuring sleep with EEG to also systematically assess alertness and performance following short naps. Further, a study which specifically investigates the relative contribution of nap length and

32

ACCEPTED MANUSCRIPT SWS to sleep inertia following short naps is needed to further explore existing findings. Studies of SWS deprivation in which participants are presented with a stimulus to arouse them from deeper sleep (81) may be useful. However, when arousing participants into a lighter sleep stage, they may unintentionally be fully awakened by this technique. In short

RI PT

naps, this sleep fragmentation could confound results by changing nap length and the number of awakenings within the nap. However, using a novel technique such as

SC

transcranial direct or alternating current stimulation, slow wave power can be enhanced (82, 83) or suppressed (84) without arousing the participant. Therefore, it might be possible

M AN U

to artificially manipulate delta power, or SWS, in a within-subject design, while controlling for nap length and timing. This approach may provide a novel insight into the role of SWS,

Conclusions

TE D

relative to nap length, in sleep inertia following short naps.

Based on studies of afternoon naps, naps shorter than 15 minutes tend to avoid sleep inertia and SWS. However, this review has highlighted that, under different conditions, the

EP

same length nap can have different sleep architecture and sleep inertia profiles. Therefore,

AC C

recommending a specific nap length to shift workers needs to be informed by details regarding time of day and prior sleep/wake conditions. It is also important to consider the purpose of the nap; that is, whether to improve immediate performance or to extend performance across several hours. Further research is needed to investigate short naps under conditions that better reflect their use in safety-critical environments. Given the myriad combinations of prior sleep/wake histories possible, and the presence of sleep inertia even under well-rested conditions, it is

33

ACCEPTED MANUSCRIPT essential to highlight, particularly in industry-focussed publications, that sleep inertia can occur at any time, after any sleep period, and that this period must be managed effectively to avoid safety critical errors so as far as reasonably practicable.

RI PT

To be clear, however, short naps can improve performance and alertness and are an important countermeasure to sleepiness. When planning on-shift naps, the risk of sleep inertia should always be considered to ensure that there are strategies in place to manage

SC

that risk.

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The higher WOE criteria outlined in the search methods for Assumption 1, together with the need for EEG sleep measures as identified in the review of Assumptions 2 and 3, inform the research design of future studies in this area, for example:

Analyse data in distinct time bins no greater than 15 minutes;



Administer test bouts frequently (at least every 15 minutes) from the point of

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waking up to at least one hour after waking in order to capture the time course of sleep inertia;

Measure a pre-nap baseline and include a no-nap control group for comparison (to

EP





AC C

control for time-of-day and sleep loss effects); and Measure sleep using electroencephalography (EEG).

Importantly, well-designed studies of sleep inertia following short naps will help to fill the gaps identified in Table 4. Furthermore, the findings from future studies, together with this review will help to inform guidelines for napping on shift by challenging the primary assumption that short naps always avoid sleep inertia.

34

ACCEPTED MANUSCRIPT

N=21 (data from N=15 who slept)

SOL at 65 min

SSS, POMSFatigue

SOL

-

ERP (P300)

SDST, LCT

TST: 3 min

0 min

SWS: Not reported (presumed none)

35

30 s/90 s: no changes. 10 min: improvements in most measures 35 min post-nap (i.e. not immediately at 5 min)

No. of correct detections on an auditory oddball task

Significant reduction in ERP amplitude (reduced stimulus arousal, reduced ability to process relevant stimuli). No change in performance.

Limitations

Main findings

Objective performance measures^

RI PT Objective alertness measures^

SC

Subjective alertness measures^

(time since nap end)

Testing points

M AN U

~15:00

No sleep details reported.

5, 35 min

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2h less than habitual

TST: 30 s, 90 s, 10 min

EP

Low

15:00

Nap duration

Bastuji et al., 2003

5-h TIB (24:00 to 05:00)

AC C

N=16

Nap timing (end at)

High

Prior sleep–wake

Participants

Tietzel & Lack, 2002

Author/s, date

Weight of Evidence

Table 1 Summary of studies investigating short, daytime naps

Only two test points within an hour.

Only one test point.

ACCEPTED MANUSCRIPT

N=10

2h less than habitual (delayed bedtime)

~14:00

5 min of Stage 1

1, 6, 11, 16, 21, 26 min

TIB: 6.5 min

Pooled into 0–15 min and 15–30 min bins.

TST: 4.5 min SWS: none

3 min of Stage 2

TST: 9.1 min SWS: none

(02:00 to 07:00)

15:00

TST: 5, 10, 20, 30 min

5, 35, 95, 155 min

SWS:

SOL at 65, 125, 185 min

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5-h TIB

5-min: 0.02 min (N=1)

EP

N=24

10-min: 0.91 min (N=5)

SDST, visual detection task

SSS, POMSFatigue

SOL

SDST, LCT, simple visual RT task

Subjective sleepiness reduced immediately after nap.

20-min: 5.71 min (N=16) 30-min: 11.37 min (N=21)

5-min: no change post-nap other than improvement on LCT 35 min postnap.

20-min: No immediate significant changes, but

36

Pooled data points in 15 min bins.

No significant improvement immediately after nap. Some improvements at 15–30 min after nap in the Stage 2 condition.

10-min: improvement on almost all measures from 5 min post-nap.

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High *

Slow rolling eye movements

M AN U

TIB: 11.4 min

Brooks & Lack, 2006

VAS

RI PT

High *

SC

Hayashi et al., 2005

Only two testing points within an hour.

ACCEPTED MANUSCRIPT

Hilditch et al., 2015

High *

N=21

24 h of wakefulness

07:10

M AN U

SC

RI PT

Brooks & Lack, 2006 (cont’d)

TIB: 10 min

2, 47 min

TST: 9.1 min

Driving simulator: 5–45 min (5 min bins)

-

PVT-B, Driving simulator

High

N=12

5 h TIB (24:00 to 05:00)

15:15

TST: 10, 30 min

AC C

Tietzel & Lack, 2001

EP

TE D

SWS: 1.3 min

SamnPerelli Fatigue Scale

SWS: 7.5 x more delta wave activity in 30 min compared to 10 min

5, 35 min SOL at 65 min

37

trend for worse alertness/ performance at 5 min, some improvements at 35 min. 30-min: sig worse at 5 min; better or no diff at 35 min.

Significant decrease in performance post-nap vs. prenap and no-nap group. No change on driving simulator.

SSS, POMSFatigue

SOL

SDST, LCT

10-min: better at 5 min and 35 min. 30-min: worse at 5 min, better at 35 min.

Only two PVT-B testing points due to driving simulator.

Only two test points within an hour.

ACCEPTED MANUSCRIPT

Takahashi et al., 1998

Low

N=30

TIB 7 h

~13:00

TIB: 15 min

30, 210 min VAS

P300 ERP, HRV

TST: 7.3 min

RI PT

SWS: none

English transcript -tion task (ETT)

TIB: 45 min

SC

TST: 30.1 min

TE D

No differences between groups for HRV. Delayed improvement in ETT following 15min nap only (interpret with caution).

EP Low

N=12

7.4 h

AC C

Takahashi & Arito, 2000

12:45

TIB: 15 min TST: 10.2 min

30, 60, 90, 120 min

SWS: None

38

First testing point 30 min after nap.

Sleepiness reduced post-15min nap vs nonap at 30 min post-nap.

M AN U

SWS: 4.7 min (N=4/10 entered)

P300 latency reduced in 15min nap at 30 min post-nap relative to 45-min nap and no-nap. Potential sleep inertia from 45min nap.

VAS

P300 ERP, HRV

Logical reasoning and digit span

Shorter P300 latency and fewer errors on logical reasoning at 30 min after nap

First testing point 30 min after nap.

ACCEPTED MANUSCRIPT

N=9

Habitual (7.8 h)

14:20

TIB: 15 min TST: 12.9 min

5, 10, 15, 20, 25 min

SC

High *

N=8 (Factory workers)

Habitual

12:45

EP

Low

TIB: 15 min

30 min

No sleep details reported.

AC C

Takahashi et al., 2004

39

No differences between groups for HRV.

Heart Auditory rate, oddball blood task pressure and doze time (Stage 1 during task)

No change in first 10 min after waking. Number of errors and amount of doze time decreased 15 min after waking. No change in subjective sleepiness.

-

9-point sleepiness scale

-

Subjective alertness was improved postnap after 4 days of napping. No change in first 3 days.

First test point 30 min after nap. Only one test point.

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SWS: none

Lower sleepiness at 30 and 60 min post-nap.

VAS sleepiness

M AN U

Kaida et al., 2003a

vs. no-nap.

RI PT

Takahashi & Arito, 2000 (cont’d)

-

ACCEPTED MANUSCRIPT

8h

Varied : every 6 h for 64 h. Results here focus on the 15:45 and 09:45 naps.

Habitual sleep

13:00

-

TST: 17.6 min (mean of day naps) SWS: 6.1 min (mean of day naps)

TIB: 20 min

0, 100 min

TST: 11.4 min (actigraphy)

TE D

N=7

-

EP

Low

1 min

AC C

Hayashi et al., 2003a

TIB: 20 min

5-min logical reasoning task

Speed, but not accuracy, worse than no-nap group post-nap.

Only one post-nap test point.

RT on visual detection task

Subjective sleepiness increased immediately after nap relative to pre-nap, but was reduced at later test points. In the no-nap condition, sleepiness increased from pre- to post-nap, and remained elevated. No change in performance or fatigue.

Only one test point post-nap.

RI PT

N=19

SC

Low

M AN U

Naitoh et al., 1993

40

VAS sleepiness and fatigue

None

ACCEPTED MANUSCRIPT

N=10

Habitual sleep

13:00

TIB: 20 min TST: 14.8 min SWS: none

2, 7, 12, 17, 22, 27, 32, 37, 42, 47, 52, 57 min

EEG 411 Hz, P3 amplitude

Memory search task (4 min)

No difference in first 15 min after waking vs. nonap, suggests no inertia. Improved alertness 15– 60 min and overall reduction in alpha-theta range (4–11 Hz) post-nap vs. nonap, but no improvement to objective performance or change in P3 amplitude.

Binned data.

Memory search task (10 min)

No difference immediately after nap on all measures. Nap then stabilised performance and alertness across the next hour relative to no-nap condition.

-

N=10

Habitual (TST 7.3 h)

14:20

TIB: 20 min

TST: 16.3 min (range 14– 19 min)

Objective: 10, 20, 30, 40, 50, 60 min

EP

High *

SWS: 2/10 entered (1 & 2 min)

AC C

Hayashi et al., 2004

TE D

M AN U

Binned into 15 min sessions (3 tests)

VAS

RI PT

High *

SC

Hayashi et al., 2003b

One woke from SWS.

VAS: 0, 10, 20, 30, 40, 50, 60 min Fatigue Questionnaire: prepost task

41

VAS: sleepiness, fatigue, motivation 34-item Fatigue Questionnaire

-

ACCEPTED MANUSCRIPT

N=12

Full night sleep, followed by 30 h of wakefulness

12:00

TIB: 20 min TST: 17.2 min

0, 15, 30, 45, 60 min

KSS

SWS: 32.4%

-

4-min 2back working memory task

(20-min nap only)

28-min visual vigilance task

Performance was improved 30 min after the nap to baseline levels (after a full night’s sleep) relative to no-

RI PT

High *

SWS at lights on: 42%

SC

Signal et al., 2012

TIB: 40 min

RT was slower immediately after waking from the 20-min nap. No significant differences were seen for other variables or at other time points.

M AN U

SWS: 57.7%

-

TST: 38.6 min SWS at lights on: 67%

TE D

TIB: 60 min TST: 57.8 min SWS: 56.9%

Gillberg et al., 1996

Low

N=8

24:00– 04:00

AC C

EP

SWS at lights on: 17%

11:15

TIB: 30 min TST: 19.8 min

30 min, 3.3 h

KSS

SWS: 3.7 min

42

Karolinska Alertness Test (KAT)

No testing within 30 min of waking: deliberately

Low

N=8 (Sleep scientists)

>7 h with wake time before 12:00 on day of study

20:15

TIB: 30 min

0 min

KSS

TST: 4.9 min (no SWS)

M AN U

Howard et al., 2010

-

SC

Gillberg et al., 1996 (cont’d)

RI PT

ACCEPTED MANUSCRIPT

PVT

nap.

avoided this period due to sleep inertia.

No difference compared to nonap immediately post-nap.

Only one test point post-nap.

AC C

EP

TE D

Notes: ^included if performed within an hour of the nap ending. *indicates inclusion in review of Assumption 3. EEG = electroencephalography; ERP = eventrelated potential; h = hour/s; HRV = heart rate variability; Hz = hertz; KAT = Karolinska Alertness Test; KSS = Karolinska Sleepiness Scale; LCT = letter cancellation task; min = minute/s; N = number; P3/P300 = ERP component with a peak at ~300 milliseconds; ETT = English transcription task; POMS = Profile of Mood States; PVT = psychomotor vigilance test; PVT-B = brief (3-min) psychomotor vigilance test; RT = reaction time; SDST = symbol-digit substitution task; SOL = sleep onset latency; SSS = Stanford Sleepiness Scale; TIB = time in bed; TST = total sleep time; s = seconds; SWS = slow wave sleep; VAS = Visual Analogue Scale/s.

43

ACCEPTED MANUSCRIPT

TST: 8 min SWS: 0.8 min

TIB: 30 min

TST: 26.4 min

SWS: 14.7 min

-

PVT-B, DSST

No change in performance pre- to post-10min nap. Significant decrease in performance post-30-min nap until 47 min post-nap. Self-rated performance showed increase in performance post-nap.

44

Limitations

Main findings

Objective performance measures^

RI PT

SC

Subjective alertness measures^

KSS, SamnPerelli Fatigue Scale, selfrated perform ance

Objective alertness measures^

(time since nap end)

2, 17, 32, 47 min

Testing points

Nap duration TIB: 10 min

M AN U

04:00

TE D

21 h of wakefulness

Nap timing (end at)

Prior sleep–wake

Participants N=32

EP

High *

AC C

Hilditch et al., 2016b

Weight of Evidence

Author/s, date

Table 2 Summary of studies investigating short, night-time naps

-

ACCEPTED MANUSCRIPT

8h

Varied: every 6 h for 64 h. Results here focus on the 03:45 and 21:45 naps.

N=24 (Aircraft mainte -nance engine -eers)

Habitual sleep

01:30– 03:30

-

TST: 18.3 min (mean of night naps)

-

5-min logical reasoning task

SWS: 7 min (mean of night naps)

TIB: 20 min TST: 19 min (self-reported) 50% reported not being able to sleep.

30 min

VAS

-

Simple RT task (2 min) Mackworth clock vigilance task (5 min)

TE D

Low

<1 min

Speed, but not accuracy, worse than no-nap group post-nap.

High *

N=12

Full night sleep, followed by 20 h of

02:00

AC C

Signal et al., 2012

TIB: 20 min TST: 17.4 min

0, 15, 30, 45, 60 min

SWS: 4.4% SWS at lights

45

KSS

-

4-min 2-back working memory task

Only one post-nap test point.

Six participants withdrew from study following the 03:45 nap due to protocol intolerance. Vigilance and subjective fatigue were worse post-nap compared to pre-nap. Reaction time did not differ.

No testing point postnap in the no-nap control condition. First testing point 30 min after nap. Only one postnap testing point.

(20-min nap only)

-

EP

Purnell et al., 2002

TIB: 20 min

RI PT

N=19

SC

Low

M AN U

Naitoh et al., 1993

No difference vs no-nap after the 20-min nap at

ACCEPTED MANUSCRIPT

wakefulness

Signal et al., 2012 (cont’d)

on: 25%

any testing point, for any variable.

RI PT

TIB: 40 min SWS: 33.8% TST: 37.1 min

TST: 55 min SWS:47%

N=14 (Oil refinery process operators)

Habitual sleep for at least 2 days

Early: 01:50 TIB: 30 min Late: 04:40

0, 15 min

TST (Early/Late): 24.5/27.5 min

EP

High

SWS (Early/Late): 9.3/11.6 min

AC C

Sallinen et al., 1998

TE D

SWS at lights on: 25%

M AN U

TIB: 60 min

SC

SWS at lights on: 75%

TIB: 50 min TST (Early/Late):

46

KSS (not reported at time 0 min)

Repeated test of sustained wakefulness

Two-choice visual RT

Performance was worse immediately after waking from all naps compared to 15 min later. The opposite was observed for the no-nap condition.

Only two testing points within an hour. Final testing point 15 min after waking and no pre-nap testing point,

ACCEPTED MANUSCRIPT

Sallinen et al., 1998 (cont’d)

High *

N=22

02:30– 03:30

TIB: 30 min TST: 16.2 min SWS: 3.3 min. N=3 entered SWS, range 4.4–10.5 min.

Full night’s sleep &2h nap in afternoon (15:00 to 17:00)

03:00

therefore cannot determine whether inertia effects have dissipated.

RI PT >10, 70, 130, 190 min

SC

(Nurses and medical scientists)

Habitual sleep

Pictorial Sleepiness Scale, VAS

-

SSS, KSS, VAS, POMSFatigue

SOL

PVT (5 min)

Performance did not change at 10 min postnap, but improved at later testing points. Subjective sleepiness was reduced 10 min post-nap.

No set time post-nap for first testing point: deliberate delay to reduce impact of sleep inertia.

SDST, LCT, PVT

Subjective sleepiness worsened 10 min postnap, recovered by 40 min postnap.

Only two testing points within an hour.

M AN U

N=9

TE D

Low

TIB: 30 min

Subjective TST: 33.3 min alertness: SWS: 17.8 min 10, 40, (91% in SWS at 100, 160, 220 min. lights on)

EP

Lovato et al., 2009

SWS (Early/Late): 15.4/19 min

AC C

Smith et al., 2007

38.1/46.6 min

Objective alertness: 75, 135, 195, 250

47

Objective performance no difference in

ACCEPTED MANUSCRIPT

min.

N=8 (Sleep scientists)

>7 h with wake time before 12:00 on day of study

04:30

TIB: 30 min TST: 23.5 min SWS: 4.4 min

0 min

RI PT KSS

-

M AN U

Low

TE D

Howard et al., 2010

Objective performance: 15, 45, 105, 165, 225 min.

SC

Lovato et al., 2009 (cont’d)

first hour, later improvements.

PVT

RT worse postnap compared to pre-nap, but no different to no-nap. Therefore, possibly due to circadian rhythm.

Only one test point post-nap.

AC C

EP

Notes: ^included if performed within an hour of the nap ending. *indicates inclusion in review of Assumption 3. h = hour/s; DSST: digit-symbol substitution task; KSS = Karolinska Sleepiness Scale; LCT = letter cancellation task; min = minute/s; N = number; POMS = Profile of Mood States; PVT = psychomotor vigilance test; PVT-B = brief (3-min) psychomotor vigilance test; RT = reaction time; SDST = symbol-digit substitution task; SOL = sleep onset latency; SSS = Stanford Sleepiness Scale; TIB = time in bed; TST = total sleep time; s = seconds; SWS = slow wave sleep; VAS = Visual Analogue Scale/s.

48

ACCEPTED MANUSCRIPT

Hayashi et al., 1999a

N=10

Hayashi et al., 1999b

N=7

7.8 h

No

Takahashi & Arito, 2000

N=12

Minimum 7-h TIB

No

No

No

~4.5 h

EP

No

~6 h

No

~5 h

No

~13:00

~12:40

% of participants in SWS at lights on

Number entering SWS

RI PT 7.3 min

0 min

0

0%

~25 min

20 min

0.8 min

2/10

20%

SC

~6 h

AC C

7.7 h

15 min

Nap duration (TIB)

No

(mean 7.4 h)

SWS duration

N=30

Nap duration (TST)

Takahashi et al., 1998

M AN U

Minimum 7-h TIB

TE D

Well-rested

Nap timing (end at)

Extended wakefulness?

Prior wake

Sleep restricted?

Prior sleep

Participants

Author/s, date

Table 3 Summary of studies describing slow wave sleep (SWS) in short naps

(3 & 5 min)

~14:30

~27 min

19.9 min

0 min

0

0%

12:45

15 min

10.2 min

0 min

0

0%

(mean 7.4 h)

49

ACCEPTED MANUSCRIPT

N=10

6.9 h

No

~7.5 h

No

14:30

-

Kaida et al., 2003a

N=9

7.8 h

No

~7 h

No

14:20

15 min

Hayashi et al., 2003b

N=10

7.3 h

No

~6 h

No

13:00

Kaida et al., 2003b

N=10

7.6 h

No

~7 h

No

14:20

Hayashi et al., 2004

N=10

7.3 h

No

~7 h

No

14:20

Fushimi & Hayashi, 2008

N=101

No

-

-

12.9 min

0 min

0

0%

14.8 min

0 min

0

0%

20 min

18.3 min

0 min

0

0%

20 min

16.3 min

0.3 min

2

10%

SC

M AN U

20 min

TE D

~5–8 h

6.6 min

12:30–15:00

8–35.5 min

EP

No

N=22

7.7 h ending at 07:00

AC C

Lovato et al., 2009

6.7 h

29 min

RI PT

Mednick et al., 2002

No

9.5 h

No

03:00

30 min

50

(1 & 2 min) 5–10 min

0 min

0/8

10.5–15 min

0 min

0/35

15.5–20 min

2.1 min

7/39

20.5–25 min

3.8 min

4/10

25.5–30 min

5.0 min

2/4

30.5–35 min

14.1 min

4/5

33.3 min

17.8 min

-

-

91%

ACCEPTED MANUSCRIPT

&

N=8 (Sleep scientists)

>7 h with wake time before 12:00 on day of study

No

>6 h

No

20:15

>16 h

Yes

04:30

Habitual. Details not provided.

No

TST: 75 min

Yes

Restricted sleep Tilley et al., 1987

N=8

(to achieve 50% of BL SWS)

~5 h

4.9 min

0 min

0

No

-

30 min

23.5 min

4.4 min

4

~13:00

~17 min

10.8 min

0 min

0

0%

~13:30

~30 min

23 min

15 min

-

-

EP

N=36

AC C

Alger et al., 2012

30 min

TE D

After at least two nights off.

SC

Howard et al., 2010

M AN U

Lovato et al., 2009 (cont’d)

RI PT

2-h nap in afternoon (15:00– 17:00) TST: 1.8 h

~13 h

No

51

ACCEPTED MANUSCRIPT

N=8

4-h TIB

Yes

7h

No

11:15

30 min

Yes

~7 h

No

~14:00

6.5 min

24:00– 04:00 3.7 h TST TST 5 h

5 h TIB

N=19

~8 h

3.7 min

-

-

4.5 min

0 min

0

0%

9.1 min

0 min

0

0%

5 min

0.0 min

1

-

10 min

0.9 min

5

20 min

5.7 min

16

30 min

11.4 min

21

17.6 min

6.1 min

-

18.3 min

7 min

M AN U Yes

7.5 h

(02:00– 07:00)

Extended wakefulness Naitoh et al., 1993

11.4 min

TE D

N=24

SC

Instructed to sleep 2 h less than habitual (delayed bedtime): actually only 1.5 h less. No

15:00

-

EP

Brooks & Lack, 2006

N=10

AC C

Hayashi et al., 2005

19.8 min

RI PT

Gillberg et al., 1996

No

22–64 h

Yes

Day (09:45, 15:45) Night (21:45, 03:45)

52

20 min

-

ACCEPTED MANUSCRIPT

N=9 (Nurses and medical scientists)

No

Habitual sleep.

No

~18 h

Yes

~21 h

~20 h

Yes

No details reported. After at least one day off.

N=12

~7 h

30 min

24.5 min

9.3 min

04:20

30 min

27.5 min

11.6 min

02:30–03:30

30 min

16.2 min (for those who slept, N=6/9)

3.3 min

No

~20 h ~30 h

Gander et al., 2010 N=21

9h TIB before simulated night shift

No

Hilditch et al., 2016b

N=32

9h TIB before simulated night shift

No

24 h

Yes

AC C

Hilditch et al., 2015

EP

Signal et al., 2012

Yes

21 h

Yes

-

-

3

-

N=5/12

25%

Range 4.4– 10.5 min

02:00

20 min

17.4 min

0.7 min

12:00

20 min

17.2 min

5.8 min

07:10

10 min

9.1 min

1.3 min

N=4/10

13%

04:00

10 min

8 min

0.8 min

N=3/10

10%

30 min

26.4 min

14.7 min

N=10/10

80%

TE D

Mulrine et al., 2012

01:50

RI PT

(Oil refinery process operators)

7.9–8.4 h before first night shift (after one day off).

SC

Smith et al., 2007

N=14

M AN U

Sallinen et al., 1998

53

42%

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Notes: BL = baseline; h = hour/s; min = minute/s; N = number; TIB = time in bed; TST = total sleep time; SWS = slow wave sleep.

54

ACCEPTED MANUSCRIPT Acknowledgements

AC C

EP

TE D

M AN U

SC

RI PT

C. Hilditch received financial support during manuscript preparation from a Helen Bearpark Memorial Scholarship and Endeavour Research Fellowship.

55

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