Ten young adult (ages 17-21) volunteers lived on a 90 min sleep-wake schedule for 86 (n = 5) or 96 (n = 5) consecutive 90 min periods. Subjects were permitted to sleep in 30 min episodes separated by 60 min of enforced wakefulness. These bedrest episodes were monitored by electroencephalogram, electro-oculogram, and electromyogram and were scored in 30 sec epochs using standard criteria. REM sleep episodes on this schedule occurred with a clear daily cycle; 74% of REM sleep time occurred during the rising phase of the body temperature cycle. REM showed a marked tendency to recur during alternate bedrest episodes. Finally, REM and slow wave sleep occurred together during only 27 of910 bedrest episodes. The findings support a circadian influence on the temporal distribution of REM sleep. Neither the sleep-independent nor the sleep-dependent models of REM sleep were supported by findings on the 90 min schedule.