I have also read a study (can find a link if somebody wants it) that observed bushmen in the kalahari (or somewhere like that - mostly untouched by modern civilisation) and found that the distribution of sleeping patterns (i.e. night owls vs early birds) was pretty uniform. They also found that because of this, there was always somebody in the tribe who was awake at any given point. They theorised an evolutionary advantage to this in that there is always somebody awake to keep watch.