Morning and night people

Some people feel most alert and energized the moment they wake up, while others' minds seem to wind up when they should be going to bed. And then there are those in-between. Sleep experts believe that there are specific biological traits in a person that, collectively, give rise to these purported personality differences.

Circadian period length:
The earth rotates on a 24.0h period. Ideally, all of us earthly beings would have a 24.0h circadian cycle. However, humans actually have a (small) range of circadian periods, and the average is believed to be slightly longer than a full day - 24.18h. The reason this isn't a major problem is that sunlight resets our clock to be on the earth's schedule.
It's thought that morning people likely have shorter circadian periods, while evening people have longer ones. People on longer circadian periods probably have a slightly delayed peak in evening alertness compared to those with short periods, and thus are more likely to sleep late. Furthermore, since they need to reset their cycle back in the morning, it may be harder to wake up. It's like a mini-jetlag.

Sleep pressure build up:
It is also believed that from the moment we wake up, our 'sleep pressure' begins to develop and increases throughout the day. The longer we are awake, the more sleep pressure there is. However, there is likely some variation in the steepness of the sleep pressure curve, that makes some people feel more tired at 10pm than others.


Source:
Czeisler et al. Science 284, 1999.