Sleep patterns. Nature or Nurture?

[TheRealJoeFish]

My understanding was that, in studies where they basically didn't let people look at the time for a week or two there was a tendency to drift towards longer day cycles of something like 27 hours. I'll see if I can find anything on that.

Another thing to note: have any of you seen those commercials for... I forget what it's called, Natural24 or something like that? They bring up a good point: a number of blind people have trouble syncing their waking/sleep schedules with the solar cycle in the absence of visual light/dark cues. EDIT: This sort of thing also appears to be a rather major problem for international airline pilots too. Essentially, it seems as though the 24-hour cycle is programmed into the human body (inasmuch as the brain and other systems are generally optimized for such a cycle) but the body is reliant on external cues to moderate that cycle. When those cues are disrupted, it results in difficulty.

Edited AGAIN to add: Wikipedia Article on Non-24, the sleep disorder I was referring to above. This includes what is generally considered to be the specific mechanism: Essentially, the day-night cycle is regulated by the pituitary gland, which left to its own devices (ie in the absence of sunlight) averages about 24.2 hours (though it's highly variable and can easily drift up or down). Normally, the sun rising every morning resets the pituitary cycle, hastening the start of the cycle if sunlight occurs before the pituitary gland has started on its own (or, similarly, darkness delaying the start of the pituitary gland is running ahead). In people who either 1) do not observe day/night differences or 2) observe day/night differences in irregular or non-circadian (that is, non-24 hour) ways, there is no proper external regulation of the pituitary gland, and that can result in, for example, a blind person's "natural" wake up time being pushed back an hour every day, which can ibviously severely disrupt social and professional life.