Summary:Many studies of nocturnal sleep, daytime naps, and phase shifts of sleep time indicate that rapid eye movement (REM) sleep has a circadian rhythm with an acrophase in the early morning, whereas slow wave sleep (SWS) correlates positively with the length of prior wakefulness. We confirmed that REM sleep has a stable circadian variation; large REM sleep amounts occurred in morning naps despite increase of SWS, owing to 1 night of total sleep deprivation. Heart rate and oral temperature both continued to show a circadian rhythm in spite of 1 night of total sleep deprivation. The lowest point of both cycles occurred in the early morning and the highest point in the late afternoon. The amount of REM sleep was largest near the low point of the circadian cycle of oral temperature and heart rate, and smallest at the high point. indicating a phase reversal relationship between the circadian rhythm of REM sleep and the autonomic functions. During 1 week of absolute bed rest under entrained conditions, subjects were most able to sleep near the low point of their oral temperature cycle and least able to sleep near the high point, and the amount of REM sleep was largest near the low point of the oral temperature and smallest at the high point.