The effects of a 20 min nap in the mid-afternoon on mood, performance and EEG activity
Introduction
It is well-known that the performance of humans declines in the afternoon (the so-called `post-lunch dip') (Blake, 1967). Daytime sleepiness and nodding off also occur at approximately 14:00–16:00 h (Dinges, 1989; Dinges, 1992). Fatigue-related accidents often occur at this time (Mitler et al., 1988). Studies examining polyphasic sleep opportunities have shown that sleep tendencies are enhanced during this period (Richardson et al., 1982; Lavie, 1986; Carskadon, 1989). The post-lunch sleepiness occurs whether lunch is consumed or not (Stahl et al., 1983), so that it is considered to be a part of a biological rhythm, such as a circadian rhythm (Broughton, 1989).
One of the countermeasures for this post-lunch sleepiness is napping. Afternoon napping is common in many countries and areas (e.g. the siesta) (Webb and Dinges, 1989). However, napping has not only positive effects such as the refreshment of mood and improvements in sleepiness or performance level, but also negative effects such as `sleep inertia,' i.e. impaired alertness usually experienced upon awaking (Lubin et al., 1976; Dinges, 1989; Dinges, 1992). These positive and negative effects of napping are dependent on the time of day the nap is taken, the duration of the nap, and prior wakefulness before napping (Naitoh, 1981).
Stampi et al. (1990)examined the effects of polyphasic daytime naps after 4 h of night sleep. These nap conditions were 3 (80 min), 5 (50 min) and 12 (20 min) naps per day. The results showed that the 50 min condition napping was least effective because the subjects were awakened from slow-wave sleep (SWS), so that their sleep inertia was increased. The 20 min condition was most effective because the subjects were awakened from lighter sleep stages, so that the sleep inertia was suppressed. The positive effects of short naps (less than 30 min) was also observed after restricted night sleep (Gillberg et al., 1996; Horne and Reyner, 1996; Reyner and Horne, 1997), and during 64 h of continuous work (Naitoh et al., 1992).
Dinges (1992)classified naps into 3 types. Replacement naps are taken in response to subjective fatigue, as a consequence of reduced nocturnal sleep. Appetitive naps are taken without regard to fatigue, as a part of an endogenous biphasic sleep cycle. Prophylactic naps are taken in advance of sustained wakefulness. Thus, the short naps studied previously (Stampi et al., 1990; Naitoh et al., 1992; Gillberg et al., 1996; Horne and Reyner, 1996; Reyner and Horne, 1997) can be regarded as replacement naps against restricted night sleep. However, daytime sleepiness does increase in the afternoon even after extended night sleep, e.g. 10 h of time in bed (Carskadon, 1989). The question arises as to whether a short daytime nap has prophylactic effects on daytime sleepiness after a full night's sleep. The present study examined the effects of a 20 min mid-afternoon nap after a full night's sleep on behavioral, subjective and physiological measures.
Section snippets
Subjects
Seven university students (3 male and 4 female) with good health participated in the study (aged 20–21 years, mean 20.6). Their sleep habits were assessed by the Sleep Habit Inventory (Miyasita, 1994) and Morning-Evening Questionnaire (Horne and Östberg, 1976). They also kept a sleep log for 1 week before the experiment. They all had normal sleep-wake habits, and did not complain of excessive daytime sleepiness or any other sleep-wake problems. They did not show any sign of being irregular
Sleep variables
The 1 week sleep logs before each experiment condition showed that the subjects slept for a mean of 441.7 min nightly before the Nap condition, and 436.0 min nightly before the No-nap condition. These values are not significantly different (t=2.19, d.f.=6, non-significant, n.s.). On the night before the experiment in the laboratory, they slept 467.8 min before the Nap condition, and 443.6 min before the No-nap condition. The at-home and laboratory sleep times were not significantly different
Discussion
The present results revealed the positive effects of a 20 min mid-afternoon nap on subjective, behavioral and physiological measures. Sleepiness, the performance of auditory vigilance and EEG alpha activity at the frontal-central area were improved for 3 h after napping. Thus, the 20 min nap during the post-lunch dip had positive effects on the maintenance of the subsequent vigilance level. It also improved the performance of the higher cognition demand tasks of logical reasoning and
Acknowledgements
This study was supported in part by the Special Coordination Funds for Promoting Science and Technology from the Science and Technology Agency of Japan, and by a Grant-in-Aid for Scientific Research funded by the Japanese Ministry of Education, Science, Sports and Culture (No. 08710047).
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