The reason grandparents get up at dawn and teenagers stay awake into the wee hours might be related to how ancient humans guarded their settlements, New Scientist reports. For 20 days, anthropologists monitored sleep patterns in the Hadza, hunter-gatherers who live in groups of about 30 individuals in modern-day Tanzania. Thanks to the wide variation in sleep schedules among young and old, at least one person was awake 99.8% of the time, researchers report this week in the Proceedings of the Royal Society B. From an evolutionary perspective, say scientists, that wide range of sleeping patterns would have been an advantage for ancient humans, ensuring that—at any one time—at least one individual would stay alert to protect the tribe in case of danger.
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