circadian cycles are advanced in older adults who tend to go to sleep earlier and wake earlier. Such age-related variation also may have been an evolutionary adaptation: It might have been a survival advantage for the older people, whose hunting skills had diminished, to stand on guard at night so that the younger, sharper hunters could get a good night’s sleep. This has led one group of researchers to propose the “poorly sleeping grandparent hypothesis.” If the sentinel hypothesis is true, we’d predict that in ancient times, only rarely would all members of a living group be asleep at the
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