Human sleep: its duration and organization depend on its circadian phase.

Two- to threefold variations in sleep length were observed in 12 subjects living on self-selected schedules in an environment free of time cues. The duration of polygraphically recorded sleep episodes was highly correlated with the circadian phase of the body temperature rhythm at bedtime and not with the length of prior wakefulness. Furthermore, the rate of REM (rapid eye movement) sleep accumulation , REM latency, bedtime selection, and self-rated alertness assessments were also correlated with the body temperature rhythm.