I thought about this a while back, and it occurred to me that if you had a tribe of primitive humans, it would actually be advantageous if not everyone had the

I thought about this a while back, and it occurred to me that if you had a tribe of primitive humans, it would actually be advantageous if not everyone had the same biological clock. It might be useful to have a few people usually awake at night, guarding the camp, instead of everyone passing out quickly after sunset.