Dynamics of nap sleep during a 40 hour period.

Following 1 baseline night, the sleep of 8 adult males in equally space 1 h naps during a 40 h period was examined. Ten additional subjects were sleep-deprived for 40 h with 1 h periods of exercise given in place of naps. One recovery night followed the 40 h period for both groups. Total sleep time and the amount of stage REM during the naps were negatively related to the circadian-temperature cycle. Stage REM frequently appeared within 10 min of stage 1 onset and the normal sequence of stages REM and 4 were altered, demonstrating that the organization of sleep within a nap is quite different from that in monophasic nocturnal sleep. Auto-correlation and cross-correlation analyses showed that the relation of sleep stages from hour to hour in normal continuous baseline sleep was altered in nap-to-nap comparison. The timing of REM onset may be controlled by a sleep-dependent ultradian clock; the clock may stop upon awakening and resume at the next sleep onset. Naps had recuperative value in terms of maintaining the normal amounts of sleep stages on the recovery night; recovery sleep for the exercise group showed typical sleep-loss effects.