The Importance of Slow-wave Sleep (sws) Dur- Ing Nocturnal Sleep Is Widely Confirmed. the Fact That the Rates of Cerebral Neural Firing

THE IMPORTANCE OF SLOW-WAVE SLEEP (SWS) DURING NOCTURNAL SLEEP IS WIDELY CONFIRMED. THE FACT THAT THE RATES OF CEREBRAL NEURAL FIRING are lowest during SWS, the length of SWS is correlated with the length of prior wakefulness, and the cerebrum is isolated from sensory input and from subcortical structures suggests that SWS is associated with tissue restitution.1 Recently, the recuperative effects of SWS have also been observed in a daytime nap. It has been found that a daytime nap that contained SWS improved perceptual learning.2,3 However, because of the homeostatic nature of SWS,4 the occurrence of SWS during a daytime nap induces a decrease of SWS in the subsequent night sleep,5 so that sleeplessness during the night occurs. In addition, SWS during a nap leads to sleep inertia,6 that is, deterioration of performance or sleepiness immediately after awakening.7,8 Stampi et al6 have shown that 5 polyphasic 50-minute daytime naps after 4 hours of night sleep caused severe sleep inertia and have stated that this occurred because participants were awakened from SWS. They also showed that the shortest nap condition (20 minutes), which was virtually without SWS (only 4%), was the most effective in facilitating performance compared with longer naps (50 and 80 minutes). Tietzel and Lack9 have reported that sleep inertia occurs after a 30-minutes nap, while this is not the case for a 10-minute nap. Short daytime naps of less than 30 minutes, which rarely contain SWS, have been shown to have positive effects on daytime alertness.10 This has been experimentally confirmed after a normal night of sleep in young adults11-17 and elderly individuals,18-21 after a restricted night of sleep,6,9,22-28 and during prolonged sustained performance.29 Table 1 shows the sleep variables of daytime short naps that have been reported in previous studies to have positive effects. These short naps are mainly composed of sleep stages 1 and 2. The shortest naps were reported by Takahashi and colleagues.16,25 After normal nocturnal sleep (7.2 hours; Takahashi et al),16 a 7.3minutes nap was the shortest, which was composed of 5.2 minutes of stage 1 and 2.1 minutes of stage 2 sleep. After restricted nocturnal sleep (3.5 hours; Takahashi and Arito),25 a 10.2-minutes nap was the shortest, which was composed of 5.5 minutes of stage 1 and 3.5 minutes of stage 2 sleep. Their results suggest that the occurrence of 5 minutes of stage 1 and 3 minutes of stage 2 sleep have recuperative effects. However, it is not clear which sleep stage mainly contributes to the recuperative effects of a daytime short nap, since these naps contained both sleep stages 1 and 2. Tietzel and Lack26 examined whether 30 seconds or 90 seconds of stage 1 sleep had recuperative effects and found that these ultrabrief naps had no recuperative power. They also found that a 10-minutes brief nap had a recuperative effect, suggesting that the recuperative power of a short nap depends on stage 2 sleep, not stage 1. However, they did not show the sleep variables of the 10-minutes nap. The present study examined which sleep stage (1 or 2) provides the recuperative effects of a nap. After restricted nocturnal sleep, daytime alertness was studied after taking a nap in which the participants were (1) awakened 3 minutes after stage 2 sleep occurred (S2-nap condition) or (2) awakened 5 minutes after stage 1 sleep occurred (S1-nap condition), and (3) after taking a rest without a nap (No-nap condition). Recuperative Power of a Short Daytime Nap With or Without Stage 2 Sleep