Feedback from Sleep—Wake Rhythm onto the Circadian Pacemaker in Humans
Human circadian rhythms have been reported to entrain to an artificial light—dark cycle with bright light over than 5,000 lx during the light period (Honma, Honma, and Wada, 1987) and to make a phase-dependent phase-shift in response to a single or daily bright light pulse (Honma and Honma, 1988; Czeisler, Kronauer, Allan, Duffty, Jewett, Brown, and Ronda, 1989; Minor, Waterhouse, and Wirz-Jusrice, 1991). These findings leads to a general agreement that the major zeitgeber or time cue for the human circadian rhythms is bright light. The zeitgeber effect of light is medicated by the retina, and probably by the retinobypothalamic tract as in the case of other mammals (Moore and Klein, 1974). On the other hand, social cues such as knowledge of time, regular contact with other persons, time schedule, et cetera, were regarded as a potent zeitgeber, especially in the earlier studies (Wever, 1979). Regular gong signals requiring the subject for micturition to collect urine sample entrained the circadian rhythm in rectal temperature as well as the sleep—wake rhythm which were otherwise free-running (Aschoff, Poeppel, and Wever, 1969). The sleep schedule either phase advanced or delayed from the time of habitual sleep was reported to shift the circadian rhythms in rectal temperature or other functions with a transient period of several days (Mills, Minors, and Waterhouse, 1978). However, the mechanism of zeitgeber effect of social factors, more generally of non-photic factors, is not well understood. Furthermore, in the previous experiments in which the entrainability of non-photic factors was examined, there were at least two pitfalls to step over carefully.
KeywordsCircadian Rhythm Phase Response Curve Circadian Pacemaker Sleep Schedule Plasma Melatonin
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