Ultrashort sleep-wake cycle: timing of REM sleep. Evidence for sleep-dependent and sleep-independent components of the REM cycle.

This study investigated the temporal structure of REM sleep in three experiments utilizing the ultrashort 7-min sleep, 13-min waking cycle. The experiments were carried out for 24 or 36 h, with and without previous sleep deprivation, under two experimental conditions of instructing subjects to fall asleep or to resist sleep. Multiple REM episodes occurred in all three experiments in the two experimental conditions, particularly during the night period. The first nocturnal REM period appeared 5 h after the nocturnal sleep gate, of which 80-100 min were accounted for by non-REM (NREM) sleep. Thereafter, REM episodes occurred periodically, with a mean inter-REM interval of 86 min. Only 14 min of this interval consisted of NREM sleep. We believe that these results suggest that although the activation of the REM oscillator is dependent on a critical accumulation of NREM sleep, once activated, it continues to function during brief periods of waking.