Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day

The interaction of homeostatic and circadian processes in the regulation of waking neurobehavioral functions and sleep was studied in six healthy youn g subjects. Subjects were scheduled to 15-24 repetitions of a 20-h rest/act ivity cycle, resulting in desynchrony between the sleep-wake cycle and the circadian rhythms of body temperature and melatonin. The circadian componen ts of cognitive throughput, shortterm memory, alertness, psychomotor vigila nce, and sleep disruption were at peak levels near the temperature maximum, shortly before melatonin secretion onset. These measures exhibited their c ircadian nadir at or shortly after the temperature minimum, which in turn w as shortly after the melatonin maximum. Neurobehavioral measures showed imp airment toward the end of the 13-h 20-min Scheduled wake episodes. This wak e-dependent deterioration of neurobehavioral functions can be offset by the circadian drive for wakefulness, which peaks in the latter half of the hab itual waking day during entrainment. The data demonstrate the exquisite sen sitivity of many neurobehavioral functions to circadian phase and the accum ulation of homeostatic drive for sleep.