Are You a Night Owl or an Early Lark?

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

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