(from: http://sleepdisorders.about.com/b/2009/ ... k.htm?nl=1)
Scientists believe that some people have a genetic predisposition to rise early, like a morning lark, or to stay up late like a night owl. These genetic tendencies may lead to subtle variance in circadian rhythms, with a desire to sleep shifted either slightly early or late.
Others may have profound shifts in their circadian rhythms causing disorders that lead to problems meeting social or professional obligations. For example, teenagers are prone to delayed sleep phase syndrome with a desire to stay up until the wee hours of the morning while sleeping in the next day. The other extreme, with early bedtimes and wake-times, may represent advanced sleep phase syndrome. Each of these may be effectively treated with gradual shifts in the sleep schedule, or with the use of melatonin or bright light therapy.
