Timing of naps: effects on post-nap sleepiness levels.

The present study investigated the effects of timing of naps on nap content and sleep inertia, and the relationship between pre- and post-nap sleepiness level and nap content. Nine subjects were tested twice on the 13 min waking-7 min resisting sleep paradigm after one night of total sleep deprivation for 24 h. The ultrashort sleep-wake paradigms started at 07.00 h and were interrupted at 15.00 and 19.00 h for 2 h naps. The 2 experimental conditions were counterbalanced across subjects and separated by a 7 day rest period. The results showed that the early nap was significantly more efficient, contained more stage 3/4, and produced less sleep inertia than the late nap. The late nap was more efficient in reducing sleepiness during the last 5 h of the experiments (23.00-04.00). Only the early nap was significantly related to pre- and post-nap sleepiness levels. Overall sleepiness level and the timing of the nocturnal sleepiness gates were significantly correlated between the two parts of the study. The results were interpreted to support the priority of the ultradian phase on prior wakefulness with respect to sleep structure.