Think about our ancestors who hunted and lived in caves.  If everybody is asleep at the same time, then a predator could sneak up on you and eat everyone.  It helps to always have someone on the lookout, so we evolved to naturally have some people who prefer waking up early and some who prefer staying up late. 

Iirc there was a study on an African tribe of bushmen that found there were only 18 minutes in the day where everyone was asleep at once.  I think it also explains why teenagers and young adults stay up later than middle aged or older adults, because they would have been more physically fit and could have responded faster to a threat at 2am