You sleep at night and are active during the day that's how things work for humans, but theoretically if a human whose parents lived on earth were to be born in another planet resembling earth but the difference was that this planet has an 8.5 hour day, what kind of changes will this person undergo?if a human who grew up on earth suddenly had to move to this planet how would his body adapt? what would be the difference between a person who moves to this 8.5 hour day planet with the person that was born there? and would the person who was born on this planet be active for 4.25 hours and then sleep for the other 4.25?
How would the human body adjust to sleep times if we were to live in a place with different day lengths?
$\begingroup$ This is a cop-out, but I think in practice, the circadian rhythm for civilized humans is only loosely coupled to the day/night cycle. Astronauts will probably spend their day indoors, with artificial light they control, which will be a bigger factor than the sun. Also, note that on Earth, there are people who live in areas such as Antarctica, where for days at a time there is no night at all. $\endgroup$– SuperbestApr 21, 2014 at 20:33
$\begingroup$ It would be pretty ineffective to adapt to an 4.25-hour rhythm. The proportion of necessary, but not directly beneficial activities like driving to work or preparing a meal would rise to a very high level. I go with ThePopMachine ... $\endgroup$– ChrKoenigJul 21, 2014 at 10:01
Humans have evolved for 24 hour days and our bodies would not adapt well to this short of sleep/wake cycles (whether or not they were born there, unless they have been there for many generations and have been able to evolve for the new time). Our bodies would still want to spend about the same amount of time sleeping and being awake.
If we tried to adjust our sleep cycles to this planet with 8.5 hour days, we would experience many of the stresses that occur during jet lag (Google Scholar search), which can cause depression. Jet lag is identical to experiencing one day which is shorter or longer than the typical 24 hours.
The sunlight helps set our circadian rhythm, but 8.5 hours would be too fast. I thought I heard for an experiment that used mice that supports this, but I still haven't found the reference yet.
Nbogard referenced the article Plasticity of the Intrinsic Period of the Human Circadian Timing System, which showed that the circadian rhythm in humans can be altered by up to 0.65 hours by the use of lights. This is consistent with what I mentioned that the sunlight helps to set our circadian rhythm.
The best way to adapt would be to use artificial lighting to keep your days about 24 hour days (artificial light during waking hours, heavy curtains during sleeping hours), as mentioned in ThePopMachine's answer.
3$\begingroup$ Now we have two contradictory answers. Some sources to back up each view would be nice. $\endgroup$– jarlemagApr 18, 2014 at 9:43
$\begingroup$ Added link for sources. Also, clarified my answer, so that it is more apparent that our answers don't really contradict. $\endgroup$ Apr 20, 2014 at 15:13
$\begingroup$ This is hard to back up with evidence, but I think jet lag has to do with synchronizing to the other people in your time zone, not the sun. Furthermore, if people on such a planet lived with 24 hour cycles anyway, would the effect on their psychology would be any different from graveyard shift workers? $\endgroup$ Apr 21, 2014 at 20:37
$\begingroup$ Hopefully it is more clear now: I meant that adjusting our sleep schedules to the new planet (of 8.5 hours in each day) would be the same physiologically as jet lag ("repeated phase shifts of the biological clock"). Does that make more sense? $\endgroup$ Apr 21, 2014 at 22:20
1$\begingroup$ @nbogard, thanks, I added it into my answer. I still need to find evidence for more extreme changes to circadian rhythm. $\endgroup$ Jul 25, 2014 at 17:08
Actually, for your example it would be pretty easy to adapt. You just have a 25.5 hour day with 12.75 hours of day and 12.75 hours of night. Except you have a 4.25 hour period in the middle of the day where you stay inside and use lights (like in the evening for most people on Earth) and a 4.25 hour period at night where you make sure your eye mask is on.
Added convenience: there are three different choices of when to call day so it's good for shift work and there's no problem with traffic. Let's move there!
$\begingroup$ Most marine animals follow a 25.5 hour "day" because of tides. It is far more important to a barnacle whether it is submerged or in the air than whether it is day or night out. (That is just the most extreme example, of course. It still matters to mobile animals) Humans easily adapt to longer day lengths, as shown in cave research, but not to shorter. $\endgroup$– user17124Oct 24, 2015 at 13:58