Do you find that you're most alert after everyone else has turned in for the night? Or are you the type to hit the deserted streets for a run at 5:30 a.m.? While all humans are essentially programmed to sleep at night and be active during the day, some people have a marked morning or evening preference. Researchers believe that owls--a term for people who like to stay up late--and larks--people who like to get up early--have their body clocks set to different schedules. The body clock, or the body's circadian rhythm, controls sleep-wake patterns by regulating body temperature and hormones such as melatonin and cortisol.
Social factors, such as having a long commute to work or staying up late to work or study, also influence a person's morning or evening preference, but the most potent thing to affect circadian rhythms by far is light.
ABC News April 15, 2004