After-effect of entrainment on the period of human circadian system.

The purpose of the present study is to examine changes of the circadian period in humans during temporal isolation, where the circadian rhythm is free-running. Twelve young males each spent 22 d alone in a temporal isolation room. Rectal temperature was continuously recorded and plasma melatonin was measured on day 3 (D3) and from days 10 to 13 (D10-D13). The light intensity in the room was less than 100 lx during the waking period. The free-run period of temperature rhythm from D3 to D10 (24.75 h) was not significantly different from those of the plasma melatonin rhythm (24.63 h) and sleep-wake cycle (24.69 h). The free-run period of temperature rhythm was 24.58 h in S-1 (D2-D6), which was gradually and significantly lengthened to 24.84 h in S-2 (D6-D10), 25.16 h in S-3 (D12-D16), and 25.18 h in S-4 (D18-D22). The free-run period of rectal temperature rhythm was steadily lengthened throughout the isolation period, which was probably due to the after-effect of previous entrainment to 24-h time cues.