This was the topic of discussion with DH the other night. We are both nightowls, pretty much always have been. For my part, my circadian rhythm or internal clock hasn't any conception of: dark outside=sleep, light outside=awake. When left to my own devices/schedule I'm happiest falling asleep around 4am-ish and waking between 11am-1pm.
My hypothesis for nightowl-ism is thus: Evolutionarily tribes or families needed or could be benefited by a nightwatchmen, someone who could alert others if danger was impending via predator or competitor. It might also serve in the capacity to learn patterns of animals who move at night, which could be lunch tomorrow if their sleep/hidey hole is known.
Someone who naturally sleeps/wakes on this type of cycle would be perfect for a position of this nature.
I don't know if a contemporary nightowl/nightwatchmen exists in some fashion in our primate counterparts.
Any others on the board who are nightowls? I shall go in hunt of supporting evidence now...