My mother used to comment on intellectualism that you need someone to design the perfect arrowhead and curvature of a spear, but you also need someone who can actually put it in the mammoth (I also propose a third person who communicates well enough to get them to stop yelling at each other).

I think sleep schedules are the same though. You need people to do the gathering during the day, but you also need someone to watch for lions all night. I think I'm very thoroughly the latter. While it does take some doing and a little caffeine, I'm much happier and better adjusted sleeping during the day every day.