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Journal of Biological Rhythms
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Period and Phase Adjustments of Human Circadian Rhythms in the Real World

Ken-ichi Honma

Department of Physiology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan

Satoko Hashimoto

Department of Physiology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan

Mitsuyuki Nakao

Graduate School of Information Sciences, Tohoku University, Sendai 980-8579, Japan

Sato Honma

Department of Physiology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan

Entrainment of the circadian rhythm has 2 aspects, period and phase adjustments, which are established simultaneously in most nonhuman circadian systems. The human circadian system is unique in its functional structure in which 2 different subsystems are involved; one is the circadian pacemaker analogous to that located in the suprachiasmatic nucleus, and the other is the oscillatory system of unknown nature that drives the rest-activity cycle. The human circadian system shows the endogenous period very close to 24 h under entrainment and less sensitive to photic stimuli than under free running, which may explain stable entrainment in the real word where natural sun lights are unpredictable in terms of the intensity and time of appearance. On the other hand, nonphotic entrainment seems to play a significant role in phase adjustment of the human circadian system. Nonphotic zeitgebers initially directed to the rest-activity cycle may affect the circadian pacemaker through feedback and/or associated LD cycles.

Key Words: circadian rhythm • human circadian rhythm clock • entrainment zeitgeber

Journal of Biological Rhythms, Vol. 18, No. 3, 261-270 (2003)
DOI: 10.1177/0748730403018003008
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