Abstract
The circadian rhythms of sleep and wakefulness and of rectal temperature were recorded in 20 subjects who spent 2-3 weeks separately in a temporal isolation unit where natural as well as social time cues were eliminated. In all subjects the circadian rhythms free-ran with a period longer than 24 hours, and in 3 subjects the rhythms were internally desynchronized. In 2 subjects the circabidian rhythm of sleep and wakefulness appeared during the course of isolation. Bright lights of 5000 1ux were demonstrated to reset the free-running circadian rhythms and to resyn- chronize the internally desynchronized circadian rhythms. Bright lights had an additional effect on the circabidian rhythm to change the periodicity to circadian. Periodic application of bright lights is strongly recommended to astronauts who spend a relatively long period in space.
