A long-standing theory in anthropology known as the “sentinel hypothesis” suggests that mammals have learned to sleep only when other members of the group are alert and able to keep watch. The theory has been shown to apply to many animal species, but not to people. Until now.
Researchers tested the sentinel hypothesis in humans by looking at the sleep patterns of a rural Tanzanian tribe. Over the nearly three weeks of the study, they found that 99.8 percent of the time, at least one adult in the tribe was awake. The adults averaged just over six hours of total sleep a night and spent nearly two-and-a-half hours awake each night after initially falling asleep. In short, they had widely different sleep schedules.
The results showed that the adults were all asleep at the same time for a mere 18 minutes during the entire 20 days of the study.
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