D.T. Max reports on the mysteries and secrets of sleep:

If we can't sleep, perhaps it's because we've forgotten how. In premodern times people slept differently, going to bed at sunset and rising with the dawn. In winter months, with so long to rest, our ancestors may have broken sleep up into chunks. In developing countries people still often sleep this way. They bed down in groups and get up from time to time during the night. Some sleep outside, where it is cooler and the effect of sunlight on our circadian rhythm is more direct. In 2002, Carol Worthman and Melissa Melby of Emory University published a comparative survey of how people sleep in a variety of cultures. They found that among foraging groups such as the Kung and Efe, "the boundaries of sleep and waking are very fluid." There is no fixed bedtime, and no one tells anyone else to go to sleep. Sleepers get up when a conversation or musical performance intrudes on their rest and intrigues them. They might join in, then nod off again.

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