Non-photic entrainment of human rest-activity cycle independent of circadian pacemaker

Abstract

Entrainment of rest-activity cycles and plasma melatonin rhythms by a forced schedule was examined in humans who were held in temporal isolation with dim light conditions (<10 lx). The sleep schedule of subjects was phase-advanced by approximately 8 h from the habitual sleep time for 8 days, which was followed by a free-running session where the subjects self-determined the times of sleep and wake up for 6 days. As a result, the rest-activity cycle was advanced by 8.2 h on the first day of free-run after the forced schedule, whereas the circadian melatonin rhythm was not significantly shifted on the last day of the schedule. The findings indicate that the forced sleep schedule of 8 days entrain the rest-activity cycle but not the melatonin rhythm. Under free-running conditions, the rest-activity cycles resynchronized with the melatonin rhythm by either phase-advance or phase-delay shifts. The direction and extent of phase-shift depended on the phase relationship between the rest-activity cycle and plasma melatonin rhythm one cycle before the phase-shift. The findings indicate the existence of an oscillatory coupling between the rest-activity cycle and the plasma melatonin rhythm. It is concluded that non-photic zeitgebers can entrain the rest-activity cycle in humans, which is independent of the circadian pacemaker.