Yes, this just-so-story is suspicious, especially because it’s not “just so”—if there were such strong selection for variability, wouldn’t you expect full coverage of the night? Some people could go to bed at four and others could wake up at three. As far as I know this does not generally happen (in the absence of electric lighting) and hence the dangers of having everybody asleep at once must be manageable.
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Yes, this just-so-story is suspicious, especially because it’s not “just so”—if there were such strong selection for variability, wouldn’t you expect full coverage of the night? Some people could go to bed at four and others could wake up at three. As far as I know this does not generally happen (in the absence of electric lighting) and hence the dangers of having everybody asleep at once must be manageable.