Effects of forced sleep-wake schedules with and without physical exercise were examined on the human circadian pacemaker under dim light conditions. Subjects spent 15 days in an isolation facility separately without knowing the time of day and followed a forced sleep-wake schedule of a 23 h 40-min period for 12 cycles, and physical exercise was imposed twice per waking period for 2 h each with bicycle- or rowing-type ergometers. As a result, plasma melatonin rhythm was significantly phase advanced with physical exercise, whereas it was not changed without exercise. The difference in phase was already significant 6 days after the start of exercise. The amplitude of melatonin rhythm was not affected. A single pulse of physical exercise in the afternoon or at midnight significantly phase delayed the melatonin rhythms when compared with the prepulse phase, but the amount of phase shift was not different from that observed in the sedentary controls. These findings indicate that physical exercise accelerates phase-advance shifts of the human circadian pacemaker associated with the forced sleep-wake schedule.

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.

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