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Effects of physical exercise on human circadian rhythms

Bright light is the principal zeitgeber for the biological clock in mammals, including humans. But there is a line of evidence that non-photic stimuli such as physical activity play an important role in entrainment. Scheduled physical activity, such as wheel and forced treadmill running, has been reported to phase-shift and entrain the circadian rhythm in rodent species. In humans, several studies have reported the phase-shifting effects of physical exercise. A single bout of physical exercise at night was demonstrated to phase-delay the circadian rhythm in plasma melatonin. However, for the entrainment of human circadian rhythm, a phase-advance shift is needed. Previously, we demonstrated that scheduled physical exercise in the waking period facilitated the entrainment of plasma melatonin rhythm to the sleep/wake schedule of 23 h 40 min. This result suggested that timed physical exercise produced phase-advance shifts. A regular physical exercise also facilitated entrainment of the circadian rhythms associated with acute phase-delay shifts of the sleep/wake and light/dark schedule. These findings suggest that physical exercise is useful to adjust the circadian rhythm to external time cues, especially for totally blind people and elderly people.