Body temperature and sleep at different times of day.

Six healthy male subjects were exposed to seven different bedtime conditions, one per week. Bedtimes were scheduled in 4-h intervals, resulting in times without sleep ranging from 16 h to 40 h. The influences of overt zeitgebers were minimized and the subjects were allowed to sleep until they felt they had slept enough. Rectal temperature was measured continuously and showed a circadian rhythm during both sleeping and waking. A fall in temperature immediately after sleep onset was noted at all bedtimes except at 0700 and 1900 h. In the majority of cases temperature rose toward the end of sleep, i.e., awakenings tended to occur during the rising phase of the circadian temperature rhythm. It was suggested that the phase of the body temperature rhythm is an important factor in determining sleep duration, mainly through terminating sleep during the high or rising parts of the rhythm.