Scientists have known for a while that our natural body rhythms don't exactly link all that well with the world around us. By placing research subjects in pitch black environemnts where there is absolutely no light whatsoever, they've been able to find that most of us actually live on a 25-hour cycle when allowed to free run. They believe this developed because over our long evolutionary history, the earth actually had longer days which over the span of millions of years has gotten shorter to our familiar 24 hour one. Light can reset this schedule however, and so with the cues of the sun and artificial lighting, on a daily basis we readjust to the 24 hour day. This predisposition to a longer daily cycle probaly also explains why people find it easier to stay up later than to get to bed earlier in trying to adjust their sleep cycle.
Some people just geneticly have longer cycles on average and sometimes I wonder if I'm one of them. I remember reading in some study somewhere citing the percentages of long cyclers in a population but I have no clue what the actual numbers are. Of course, it's just generally all confounded to heck with the fact that I generally prefer the night to the day anyway. Still, it would be nice to just be able to blame everything on my genes. God knows that's the way it seems things are going nowadays.