CONTRIBUTION OF THE CIRCADIAN PACEMAKER AND THE SLEEP HOMEOSTAT TO SLEEP PROPENSITY, SLEEP STRUCTURE, ELECTROENCEPHALOGRAPHIC SLOW WAVES, AND SLEEP SPINDLE ACTIVITY IN HUMANS

The role of the endogenous circadian pacemaker in the timing of the sl eep-wake cycle and the regulation of the internal structure of sleep, including REM sleep, EEG slow-wave (0.75-4.5 Hz) and sleep spindle act ivity (12.75-15.0 Hz) was investigated. Eight men lived in an environm ent free of time cues for 33-36 d and were scheduled to a 28 hr rest-a ctivity cycle so that sleep episodes (9.33 hr each) occurred at all ph ases of the endogenous circadian cycle and variations in wakefulness p receding sleep were minimized. The crest of the robust circadian rhyth m of REM sleep, which was observed throughout the sleep episode, was p ositioned shortly after the minimum of the core body temperature rhyth m. Furthermore, a sleep-dependent increase of REM sleep was present, w hich, interacting with the circadian modulation, resulted in highest v alues of REM sleep when the end of scheduled sleep episodes coincided with habitual wake-time. Slow-wave activity decreased and sleep spindl e activity increased in the course of all sleep episodes. Slow-wave ac tivity in non-REM sleep exhibited a low amplitude circadian modulation which did not parallel the circadian rhythm of sleep propensity. Slee p spindle activity showed a marked endogenous circadian rhythm; its cr est coincident with the beginning of the habitual sleep episode. Analy ses of the (nonadditive) interaction of the circadian and sleep-depend ent components of sleep propensity and sleep structure revealed that t he phase relation between the sleep-wake cycle and the circadian pacem aker during entrainment promotes the consolidation of sleep and wakefu lness and facilitates the transitions between these vigilance states.