
Sleep is one of the great mysteries and joys of life—and a bad night’s sleep, or a series of them, is one of its great miseries. Sleep is pretty much a universal in the animal kingdom reaching down to very primitive forms. The many variations of the sleep-wake cycle all undoubtedly evolved as different and creative adaptations to the day-night rhythm of our planet.
While all humans must sleep for a substantial fraction of each day, there are wide individual differences in sleep quantity and quality. This is probably no accident and serves its own adaptive purpose. A tribe would benefit from having very different types of sleepers—from early birds to night owls—so that there would be a continuity of lookouts, night watchmen, and fire tenders. The fitfulness of our sleep...
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