I stopped taking the book seriously when I reached Walker’s suggestion that teenagers might have a sleep cycle offset from adults because “wise Mother Nature” was giving them the chance to develop independence from the tribe, in a group of their peers, and that this was an important stage in societal development of a human.
If one *must* find an evo-psych explanation for this phenomenon, surely “we need people guarding the camp at more hours of the day” is simpler and less ridiculously tenuous. (Though this still has precisely the same “I could have explained anything with this” flavour that most popular evo-psych does.)
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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.