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A disclaimer is that I haven't read "Why We Sleep". Nonetheless I recommend the book "Sync" by Steven Strogatz (2003) -- particularly chapter 3 on sleep -- which describes a neat coupling of when we sleep / when we wake up to our body temperature which oscillates with a period of roughly 24 hours. (See figures on pgs 81 and 83.) Much of this discussion on sleep is derived from "isolation" research in the mid 1970's by Elliot Weitzman and Charles Czeisler, experiments in which subjects were isolated from all sense of time including daylight, clocks, and the news, to study their sleep cycles in the absence of external information telling them the time of day.