11. Our Biological Clock And Sleep

Humans are diurnal creatures, that is, they conduct their conscious activities during the day. We are instinctively sleepers by night. The pattern of night/day sleeping continues even in subjects who are continuously kept in the dark for weeks at a time. The circadian rhythm is not disturbed by unceasing darkness. Experiments with subjects I kept in rooms without lighting for up to two weeks did not shake the rhythm, though subjects have had their rhythms lengthened and shortened while staying in caves lit with artificial daylight. The change in rhythm they experienced corresponded with the shortened or extended "days."

Our sleep patterns gradually change when we change time zones, or when normal activities are changed by advancing or regressing the clock.