Biological Psychology 83 (2010) 15–19
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The effect of self-awakening from nocturnal sleep on sleep inertia Hiroki Ikeda 1, Mitsuo Hayashi * Department of Behavioral Sciences, Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 November 2008 Accepted 17 September 2009 Available online 1 October 2009
The present study examined the effects of self-awakening on sleep inertia after nocturnal sleep. Ten undergraduate and graduate students participated in the study. Their polysomnograms were recorded for ﬁve consecutive nights; the ﬁrst, second, and third to ﬁfth nights were adaptation, forced-awakening, and self-awakening nights, respectively. Participants rated sleepiness, fatigue, comfort, and work motivation, and these ratings were followed by switching (7 min) and auditory reaction time tasks (6 min), both before bedtime (15 min) and immediately after awakening (4 min 15 min). Although reaction times on the auditory were task prolonged, and participants complained of feeling uncomfortable immediately after forced-awakening, reaction times were shortened after selfawakening, and the participants did not complain of feeling uncomfortable on these nights. The results of this study suggest that sleep inertia occurs after forced-awakening and that it can be prevented by self-awakening. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Self-awakening Sleep inertia Polysomnograms Forced-awakening
1. Introduction The ability to awaken at a predetermined time without external assistance is known as self-awakening (Zepelin, 1986). Surveys have indicated that many people can self-awaken from nocturnal sleep. Child (1892) reported that 59% of 200 people surveyed claimed that they are able to self-awaken. According to Bell (1980), Clauser (1954) surveyed the sleep–wake habits of 1080 people, 8% of those surveyed claimed that they always self-awakened, while 10% reported that they usually did so. Moorcroft et al. (1997) demonstrated that 52% of a sample of 269 adults (21–84 years old) could routinely self-awaken. In addition, Matsuura et al. (2002) found that 10.3% of a sample of 643 undergraduate students could habitually self-awaken. Self-awakening appears to have several beneﬁts. Kaida et al. (2005) found that the blood pressure of elderly participants who were asked to self-awaken from a short nap increases gradually during the 2 min before a predetermined awakening time. In contrast, blood pressure increased rapidly after forced-awakening by the experimenter. These results suggest that self-awakening prevents rapid increases of blood pressure upon awakening. Matsuura et al. (2002) reported another beneﬁt of self-awakening, namely that those who habitually self-awaken felt subjectively better in the morning in comparison to those who had no such habit. Born et al. (1999) found that adrenocorticotropin (ACTH)
* Corresponding author. Tel.: +81 82 424 6582; fax: +81 82 424 0759. E-mail address: [email protected]
(M. Hayashi). 1 Research Fellow of the Japan Society for the Promotion of Science. 0301-0511/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.biopsycho.2009.09.008
levels increase 1 h before awakening, when participants intend to self-awaken. Allen (2001) noted that secretion of ACTH might help participants to wakeup more easily and suggested that selfawakening may reduce sleep inertia, which is the state of lowered arousal that occurs immediately after awakening (Ferrara and De Gennaro, 2000; Tassi and Muzet, 2000). Kaida et al. (2003a,b) studied self-awakening from a short afternoon nap, and found that self-awakening reduced post-nap sleep inertia compared to forced-awakening by the experimenter. However, relatively little is known about the effects of self-awakening on sleep inertia immediately following nocturnal sleep. The aim of the present study was to examine the effects of selfawakening on the sleep inertia that can occur after nocturnal sleep. We used a simple auditory reaction time task that reﬂected arousal levels to measure the degree of sleep inertia. We also had participants perform a switching task that measured the ability to switch attention from one stimulus to another. This ability is one of the ‘‘executive control processes’’ that are thought to depend on frontal cortex functioning (Aron et al., 2004). It has been reported that reactivation occurs in the thalamus, caudate, brainstem and anterior cingulated cortex immediately after awakening from sleep, whereas recuperation from sleep inertia is related to reactivation at the prefrontal cortex (Balkin et al., 2002). Therefore, it is possible that the ability to switch attention is diminished by sleep inertia. Finally, we measured subjective sleepiness, fatigue, comfort levels and task motivation using the Visual Analog Scales (VAS; Monk et al., 1985). It is known that sleep inertia affects subjective sleepiness (Jewett et al., 1999). Therefore, the present study used subjective measures and an attention-switching task to examine the effects of self-awakening on post-nocturnal sleep inertia.
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2. Methods 2.1. Participants Ten undergraduate and graduate students in good health (7 females and 3 males; mean age = 21.8 0.6 years) participated in the study. All participants were free from sleep–wake problems and from habits such as daytime napping, exercising excessively, and drug use. Participants had not worked night shifts, or traveled across different time zones less than one month prior to the experiment. None of our participants were in the habit of self-awakening. All stated that they use an alarm clock to awaken, and they either woke up before the alarm goes off, or woke up upon hearing the alarm. All participants provided their informed consent. The relevant ethics committee of Hiroshima University approved the study protocol. 2.2. Procedure The study was conducted between January and April in 2006. Participants were asked to adhere to their habitual sleep–wake schedule for one week prior to the experiment. Participants’ sleep–wake schedules were monitored using sleep logs and wrist actigraphy (Actiwatch AW64, Mini-Mitter Co. Inc., Bend, Ore.), conﬁrming that their typical sleep–wake habits were kept. Participants were also asked to abstain from substances that would affect sleep and wakefulness (e.g., alcohol, caffeine and nicotine) on the experimental days. The experiment was conducted over the course of ﬁve consecutive nights. The ﬁrst night was an adaptation night. The second night was a forced-awakening night, and the remaining nights were designated as self-awakening nights. The order of the forced- and self-awakening conditions was not counterbalanced; given that prior experience with self-awakening might be expected to result in self-awakening on the forced-awakening night. Retiring and awakening times on the experimental days were set to participants’ average times during the week prior to the experiment. The participants arrived at the sleep laboratory approximately 2 h before bedtime. Then, electrodes were attached and participants then entered a soundproof and air-conditioned isolation unit. Time cues, including natural light, external noise, and temperature, were effectively eliminated in this unit. Background noise level in the unit was set to 30–31 dB (A). Illumination of the unit was set to 240 lx while participants performed the experimental tasks. The temperature of the sleep laboratory was set to 23–25 8C. Participants performed pre-sleep tasks that took approximately 15 min to complete (completing the Visual Analog Scales = 1 min; switching task = 7 min; auditory simple reaction time task = 6 min). All of these tasks were completed approximately 45 min before bedtime. Then participants were informed their time to awaken in next morning. Furthermore, on the adaptation and forced-awakening nights, participants were instructed to avoid waking up by themselves and to sleep until the experimenter called their names over an intercom. On the self-awakening nights, participants were informed the time at which they were expected to wakeup, immediately before they retired for the night. They were instructed to wakeup without help and to notify the experimenter when they had done so. At the usual retiring time for each participant, lights were turned off and polysomnogram recording commenced. On the adaptation and forced-awakening nights, the experimenter woke the participants using an intercom. On the selfawakening nights, participants attempted to wakeup at a predetermined awakening time. If they were not awake after 30 min from the predetermined time, the experimenter immediately woke them. On both forced- and selfawakening mornings, participants performed post-awakening experimental tasks for 60 min (4 task sessions of 15 min each). 2.3. Task In the simple auditory reaction time task, participants pressed a button as quickly as possible when they heard an auditory stimulus (65 dB) that was presented at random intervals of 2–8 s (mean = 5 s). Performance indices that were measured included the mean reaction time for responding and the number of lapses (no response within 2 s). The switching task was based on the task used by Kramer et al. (1999). This task was composed of two separate components, an element number component and a digit value component. Participants judged the number of digits presented in the former, whereas they identiﬁed the values of the digits in the latter. All the trials for both the tasks had a single row of digits presented on a computer screen. Each row was composed of digits of the same value, from 1 to 9 (not including 5), and the number of digits presented ranged from 1 to 9 (not including 5). Participants were instructed to judge whether the number of digit elements in the row (element number task) or the value of the digits (digit value task) was above or below 5. When the digit value or element number was higher than 5, the participant had to press the ‘‘6’’ key on the keyboard. When values were lower than 5, they pressed the ‘‘4’’ key. For example, if ‘‘777’’ was displayed on the screen, participants had to push the ‘‘6’’ key during the digit value task but the ‘‘4’’ key during the element number task. For the initial series of trials, one of two phrases (‘‘element number task’’ or ‘‘digit value task’’) was displayed on the computer screen for 2000 ms. Four hundred milliseconds later, the task trials started. Participants were instructed to press the
correct key as quickly and accurately as possible. If they did not press the key within 5 s for a given trial, an alarm sounded until the key was pressed. The inter-trial interval was set to 100 ms. After a randomly determined 5–11 trials of a given task, instructions for the other task were presented on the screen for 1000 ms, and after a further 200 ms, trials for this task began. Reaction times of the ﬁrst trials after the switch (ﬁrst trial RT) and the ﬁnal trials after the switch (last trial RT) were used to calculate the switch cost. The time required to complete the executive control processes that underlie task switching is inferred from the increased response times that are observed when a task switch occurs, as compared with response times during the same task performed separately or during a run of trials of the same task (Kramer et al., 1999). Therefore, the switch cost was calculated by subtracting the last trial RT on several trials of each block from the ﬁrst trial RT on several trials of each block. The error rate for the ﬁrst trial of each task block was also measured. Mean switch cost and error rate values were calculated within the sessions. 2.4. Subjective measures Participants rated subjective sleepiness, fatigue, comfort level, and motivation to engage in the experimental tasks using the Visual Analog Scales, before bedtime and 1, 16, 31 and 46 min. after awakening. VAS values ranged from 0 (‘‘very alert,’’ ‘‘very vigorous,’’ ‘‘very bad,’’ and ‘‘very unmotivated’’) to 100 (‘‘very sleepy,’’ ‘‘very fatigued,’’ ‘‘very good,’’ and ‘‘very motivated’’). 2.5. Polysomnogram Standard polysomnograms were recorded, to evaluate sleep variables across conditions. Electroencephalogram (EEG; C3, C4, O1, O2), horizontal electrooculogram (EOG), submentalis electromyogram (EMG), chest electrocardiogram (ECG), and respiration curve derived by nasal thermistor were measured. Time constants were set to 0.3, 2.0, 0.003, 1.0 and 2.0 s, and low pass ﬁlter did 60, 15, 300, 15 and 15 Hz in EEG, EOG, EMG, ECG, and respiration curve, respectively. 2.6. Analysis Instances in which a participant woke up within 30 min of the predetermined wakeup time were judged to be examples of successful self-awakening. Data for both forced-awakening and successful self-awakening nights were analyzed. To calculate sleep variables, sleep stages were scored in 20 s epochs using the standard criteria (Rechtschaffen and Kales, 1968), as well as the supplements and amendments to Rechtschaffen and Kales’ (1968) criteria provided by the Sleep Computing Committee of the Japanese Society for Sleep Research (2001). Sleep variables measured on these nights were compared using paired t-tests. Two-way (condition: forced-awakening versus self-awakening) session (pre-sleep session, four sessions after awakening) repeated measures ANOVAs were used to analyze task performance and subjective ratings. Degrees of freedom were adjusted via the Huynh-Feldt epsilon calculation. Post-hoc comparisons were performed using Tukey’s HSD procedure.
3. Results Table 1 shows participant bedtimes, target times (habitual awakening times) and awakening times on the self-awakening nights. Nine participants successfully self-awakened on 1 of the 3 self-awakening nights (7 females and 2 males; mean age = 21.8 0.6 years). Three of these nine participants selfawakened on the ﬁrst night, two participants self-awakened on the second night, and four participants did so on the third night. The data from one participant who could not self-awaken were excluded from the analyses of task performance and subjective ratings. Polysomnograms were not recorded for one participant, due to technical problems. Sleep variables were analyzed for the other 8 participants (6 females and 2 males; mean age = 21.8 0.6 years). 3.1. Task performance Fig. 1 shows mean RTs for the simple auditory reaction time task. The main effect of condition, F (1,8) = 8.644, p < .05, e = 1.000, and the interaction between condition and time, F (4,32) = 4.201, p < .01, e = 1.000, were both signiﬁcant. In the forced-awakening condition, reaction times were signiﬁcantly prolonged in the ﬁrst post-sleep session compared to the pre-sleep session, p < .05. In addition, reaction times were signiﬁcantly shorter in the selfawakening condition than in the forced-awakening condition, in
H. Ikeda, M. Hayashi / Biological Psychology 83 (2010) 15–19
Table 1 Bedtimes, target times and awakening times on self-awakening nights (n = 10). Participant no.
Predetermined rise time
01 02 03 04 05 06 07 08 09 10
0:30 1:00 1:30 0:00 1:00 1:00 0:30 1:00 1:30 0:00
8:00 7:30 9:00 7:00 9:00 9:00 7:30 9:00 8:00 7:00
Self-awaked time 1st night 8:30 7:58 4:06 6:30 9:30 9:33 5:29 7:01 4:36 7:26
2nd night (+30) (+28) ( 294) ( 30) (+30) (+33) ( 121) ( 119) ( 204) (+26)
5:13 4:51 9:30 7:42 8:54 8:16 5:56 9:30 8:28 8:10
8:21 8:00 9:04 3:50 9:37 9:01 6:33 8:56 3:41 7:30
( 167) ( 159) (+30) (+42) ( 6) ( 44) ( 94) (+30) (+28) (+70)
(+21) (+30) (+4) ( 190) (+37) (+1) ( 57) ( 4) ( 259) (+30)
Underline: successful nights of self-awakening; (): time gap between awakening time and predetermined rise time, calculated by subtracting the predetermined rise time from the self-awakening time.
(4,32) = 4.021, p < .05, e = .677, were signiﬁcant. In the forcedawakening condition, subjective comfort was signiﬁcantly lower for all of the times after awakening, as compared to before sleep, ps < .05. Although subjective fatigue was signiﬁcantly lower in the self-awakening condition than in the forced-awakening condition, F (1,8) = 5.857, p < .05, e = 1.000, no signiﬁcant differences between the conditions were observed for task motivation or subjective sleepiness. No signiﬁcant interactions between condition and time were observed for fatigue, task motivation, and sleepiness. 3.3. Sleep variables Fig. 1. Reaction times on simple auditory reaction time task before and after nocturnal sleep in the forced- and the self-awakening conditions. The vertical lines are SEMs; *p < .05, **p < .01.
the ﬁrst post-sleep session, p < .01. There were no signiﬁcant main effects on the lapses, nor was the two-way interaction signiﬁcant. For the switching task, there were signiﬁcant main effects of time on ﬁrst trial RTs, F (4,32) = 3.054, p < .05, e = 1.000, last trial RTs, F (4,32) = 3.831, p < .05, e = .637, and the error rate, F (4,32) = 3.092, p < .05, e = 1.000. Post-hoc comparisons showed that both last trial and ﬁrst trial RTs were prolonged in the ﬁrst post-sleep session compared to the pre-sleep session, p < .05. 3.2. Subjective measures Fig. 2 shows mean subjective comfort levels in the forced- and self-awakening conditions. The main effect of time, F (4,32) = 4.377, p < .05, e = .476, and the interaction between condition and time, F
Table 2 shows sleep variables on the forced- and selfawakening nights. Participants slept for 431 and 433 min on the forced- and self-awakening nights, respectively. Although REM sleep duration tended to be shorter on the self-awakening nights, while latencies to sleep Stages 1 and 4 tended to be longer on these nights, ps < .10, there were no signiﬁcant sleep variable differences between the self- and forced-awakening nights.
Table 2 Sleep variables on the forced-awakening (FA) and the self-awakening (SA) nights (n = 8). Sleep variables
11.6 8.9 1.3
0.96 0.29 1.76
n.s. n.s. n.s.
16.5 21.3 220.0 47.1 30.9 98.8 8.6
6.0 3.2 7.8 5.2 6.6 2.8 1.4
1.70 0.27 0.75 0.45 0.04 2.08 0.32
n.s. n.s. n.s. n.s. n.s. <.10 n.s.
0.5 0.7 1.8 1.2 1.6 1.0 0.2
3.5 4.7 49.8 10.5 7.0 22.4 1.9
1.3 0.6 2.0 1.0 1.5 1.0 0.3
1.68 0.01 0.67 0.66 0.02 1.88 0.21
n.s. n.s. n.s. n.s. n.s. n.s. n.s.
2.3 3.1 3.5 4.7 12.5
10.4 16.0 24.0 32.8 86.4
3.6 4.3 4.4 4.9 13.1
2.17 1.72 1.31 2.23 1.21
<.10 n.s. n.s. <.10 n.s.
Time in bed (min) Total sleep time (min) Sleep efﬁciency (%)
436.1 430.5 96.4
11.4 9.5 0.6
443.1 432.8 94.6
Wake after sleep onset (min) Stage 1 (min) Stage 2 (min) Stage 3 (min) Stage 4 (min) Stage REM (min) Movement time (min)
7.8 20.8 210.9 50.0 30.6 107.9 8.3
2.4 3.3 10.5 5.0 7.0 3.8 0.9
1.7 4.7 48.3 11.5 7.1 24.8 1.9 5.6 12.0 20.4 28.4 80.6
% % % % % % %
Fig. 2. Subjective comfort before and after nocturnal sleep in the forced- and selfawakening conditions. The vertical lines are SEMs; *p < .05.
Wake Stage 1 Stage 2 Stage 3 Stage 4 REM Movement time
Latency Latency Latency Latency Latency
to to to to to
Stage Stage Stage Stage Stage
1 (min) 2 (min) 3 (min) 4 (min) REM (min)
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Table 3 Number of participants who awakened at sleep Stages 1, 2 and REM, and reaction times on simple auditory reaction time task/subjective comfort immediately after awakening (on the ﬁrst 15-min post-sleep session) from these sleep stages. Sleep stage before awakening
Condition Forced-awakening Stage 1
Number of participants Reaction time of ASRT (ms)a Subjective comfortable
3 19.6 25.0
Stage 2 3 46.3 5.7
Self-awakening REM 2 45.2 14.5
All participants 8 36.0 13.4
Stage 1 1 22.6 1.0
Stage 2 5 24.6 3.2
REM 2 25.9 7.5
All participants 8 19.0 3.8
ASRT: auditory simple reaction time task. a Data are expressed as the change values calculated by subtracting the 15-min pre-sleep session from the ﬁrst post-sleep session.
3.4. Sleep stages at awakening The numbers of participants who awakened during sleep Stages 1, 2, and REM are shown in Table 3. On the forcedawakening night, 3, 3, and 2 participants awoke from sleep Stages 1, 2, and REM, respectively. In the self-awakening condition, the corresponding ﬁgures were 1, 5, and 2 participants. A relatively large number of participants awoke from sleep Stage 2 on the selfawakening nights. Simple auditory reaction times and subjective comfort ratings immediately after awakening from these sleep stages are also shown in Table 3 (for the ﬁrst post-sleep session). These data are expressed as change values from the pre-sleep session, calculated by subtracting the pre-sleep session values from those immediately after awakening. After awakening from either Stage 2, or REM sleep, reaction times were shorter after self-awakening compared to forced-awakening. These measures are almost identical when Stage 2 and REM sleep are compared. After waking up from Stage 1 sleep, reaction times were similar for the forced- and selfawakening nights. Subjective comfort tended to deteriorate after waking up from Stage 1, or REM sleep, under conditions of forcedawakening. 4. Discussion The present study examined the effects of self-awakening from nocturnal sleep on sleep inertia. After forced-awakening, reaction times on an auditory reaction time task were relatively prolonged and the participants noted that they were uncomfortable, suggesting that sleep inertia does occur after forced-awakening. This was not the case for self-awakening; in this case, auditory reaction times were signiﬁcantly shorter immediately compared to those after forced-awakening. In addition, subjective comfort did not decrease after self-awakening. Taken together, these results suggest that self-awakening can reduce post-nocturnal sleep inertia. Although REM sleep durations tended to be shorter and latencies to sleep Stages 1 and 4 tended to be longer on the self-awakening nights, various sleep variables were not signiﬁcantly different across the forced- and self-awakening nights. It has been suggested that the intention to self-awaken at a target time can sometimes cause psychological stress, such that the quality of nocturnal sleep might deteriorate (Moorcroft et al., 1997). In the study of Lavie et al. (1979), sleep latencies were extended when participants were asked to self-awaken. These ﬁnding suggests that asking our participants to self-awaken might have put them under some psychological stress. Sleep inertia adversely affects arousal levels and subjective comfort after forced-awakening. Although sleep inertia increases upon awakening from restricted nocturnal sleep, or under conditions of partial sleep deprivation, relative to normal nocturnal sleep as was experienced by our participants (Cavallero and Versace, 2003; Ferrara et al., 1999; Tassi et al., 2003, 2006),
some degree of sleep inertia still occurs after normal nocturnal sleep (Ikeda and Hayashi, 2008; Jewett et al., 1999). The present results support these ﬁndings. However, there was no signiﬁcant effect of sleep inertia on a switching task that required frontal lobe functioning, suggesting that frontal lobe functions might not be impaired immediately after awakening if participants have slept for more than 7 h. Sleep inertia is inﬂuenced by factors such as circadian rhythms (Dinges et al., 1985), sleep times (Balkin and Badia, 1988; Cavallero and Versace, 2003), amount of slow wave sleep (SWS) (Ferrara and De Gennaro, 2000; Ferrara et al., 2001, 2002), and awakening times (Dinges et al., 1985, 1987). These factors were held constant across forced- and self-awakening nights in the present study. Sleep stage before awakening is also considered to be a factor that affects sleep inertia. Sleep inertia appears to be most severe when a person wakes up from sleep Stages 3 and 4 (i.e., SWS). It has also been suggested that sleep inertia is more severe after waking up from Stage 2 sleep as compared to REM sleep (Cavallero and Versace, 2003), although this effect has not always been observed (Jewett et al., 1999). SWS rarely occurred during the latter half of our participants’ nocturnal sleep, therefore, it can be assumed that SWS did not inﬂuence sleep inertia in the present study. However, auditory reaction times and subjective comfort ratings were nearly identical after awakening from Stage 2, as opposed to REM sleep. Our ﬁndings therefore support those of Jewett et al. (1999) study, in which participants had 8 h of sleep. In the study of Cavallero and Versace (2003), sleep times were restricted. The effects of sleep stage on sleep inertia-related performance deﬁcits would appear to depend on sleep length; that is, the shorter the sleep duration, the more robust the effects of sleep stage on sleep inertia. Auditory reaction times and subjective comfort levels did not deteriorate after self-awakening. Results of the present study suggest that self-awakening does not produce sleep inertia after nocturnal sleep. These results are consistent with the ﬁndings of Kaida et al. (2003a,b), who examined the effects of self-awakening on sleep inertia after a short afternoon nap. They found that subjective sleepiness was lower and P300 event-related potentials were larger after self-awakening, compared to forced-awakening. However, the sleep length in their study (14 min) was quite different from that examined in the present study (7 h). Although sleep inertia seems to occur for about 5 min after forcedawakening from a short nap (Hayashi et al., 2003), the quality or quantity of the sleep inertia that occurs during the daytime after a short nap may be different from that which occurs in the morning after nocturnal sleep. A number of mechanisms might account for the preventive effect of self-awakening on sleep inertia. Kura¨uchi et al. (2004) found that there is a functional relationship between distal vasoconstriction at the extremities (cooling out rate) and dissipation of sleepiness after nocturnal sleep (23:00–07:00, 8 h) and an afternoon nap (16:00–18:00, 2 h). Kaida et al. (2003a,b, 2005) reported that heart rate and blood pressure gradually increase before awakening, when participants intend to self-awake, and Born et al. (1999) found that
H. Ikeda, M. Hayashi / Biological Psychology 83 (2010) 15–19
ACTH levels increase around 1 h before awakening, at least when participants intend to self-awaken. These results suggest that sympathetic nervous system (SNS) activity increases before selfawakening. The intention to self-awaken would appear to promote increased SNS activity that commences at least 1 h before awakening. This SNS activity in turn promotes distal vasoconstriction, enhancing the cooling out rate of the extremities and helping to dissipate sleepiness after awakening. This sequence of physiological processes may account for the lack of sleep inertia observed in the present study. Another possibility is that the intent to selfawaken serves to attenuate various sleep maintenance functions. Ikeda and Hayashi (2008) reported that sigma band power, which reﬂects sleep spindle activity, gradually decreases during sleep Stage 2, before self-awakening, suggestive of one desirable maintenance function that is attenuated by self-awakening, given that sleep spindles during sleep Stage 2 do serve to maintain sleep (Ueda et al., 2001). The present study has a number of limitations. Firstly, the small sample size may have limited the power of our statistical analyses. Secondly, the number of awakenings from sleep Stage 1 differed across the forced- and self-awakening conditions. This difference may have affected sleep inertia. Thirdly, the fact that the conditions were not counterbalanced means that order effects cannot be ruled out. Finally, although self-awakening does not produce sleep inertia, sleep variables tend to deteriorate as a function of selfawakening. However, 10–50% of people habitually self-awaken (Clauser, 1954; Child, 1892; Moorcroft et al., 1997; Matsuura et al., 2002), suggesting that any adverse effects may be negligible for those who make a regular habit of doing so. Therefore, participants who do not make a habit of self-awakening might be able to obtain the beneﬁts of doing so (and avoid the drawbacks) by developing the habit. Further studies of the effects of self-awakening on sleep inertia should compare those who habitually self-awaken with those who do not. In conclusion, the present study experimentally conﬁrmed that self-awakening rarely disturbs nocturnal sleep but does reduce sleep inertia immediately after awakening. Furthermore, Matsuura et al. (2002) showed that those who habitually self-awaken feel better in the morning and doze off less during the daytime than those who do not self-awaken. Developing the habit of selfawakening might help individuals to avoid the deleterious effects of sleep inertia and may improve daytime functioning. Further experimental studies are required to examine the effect of selfawakening from nocturnal sleep on sleep inertia. References Allen, R.P., 2001. Article reviewed: timing the end of nocturnal sleep. Sleep Medicine 2, 69–70. Aron, A.R., Monsell, S., Sahakian, B.J., Robbins, T.W., 2004. A componential analysis of task-switching deﬁcits associated with lesions of left and right frontal cortex. Brain 127, 1561–1573. Balkin, T.J., Badia, P., 1988. Relationship between sleep inertia and sleepiness: cumulative effects of four nights of sleep disruption/restriction on performance following abrupt nocturnal awakenings. Biological Psychology 27, 245–258. Balkin, T.J., Braun, A.R., Wesensten, N.J., Jeffries, K., Varga, M., Bladwin, P., Belenky, G., Herscovitch, P., 2002. The process of awakening: a PET study of regional brain activity patterns mediating the re-establishment of alertness and consciousness. Brain 125, 2308–2319. Bell, C.R., 1980. Awakening from sleep at a pre-set time. Perceptual and Motor Skills 50, 503–508. Born, J., Hansen, K., Marshall, L., Mo¨lle, M., Fehm, H.L., 1999. Timing the end of nocturnal sleep. Nature 397, 29–30.
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