[troutwine]

It's worth noting that some humans are shifted into the evening. Morning-persons and night-owls seem to have held different advantages to our species once we domesticated dogs. Morning folks catch the first rays of light for the day's labor, night folks tend the watch fires and keep intruders away while the bulk of the humans sleep.

[brass_cannon]

That's an interesting idea. Is there any additional reading related to this?

[troutwine]

Of course. The following Wikipedia article has several references: http://en.wikipedia.org/wiki/Chronotype It is also worthwhile to do a Google Scholar search with the query "evolution eveningness".