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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?

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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. –  Superbest Apr 21 at 20:33
    
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 ... –  Hav0k 2 days ago
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2 Answers 2

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!

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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. The sunlight helps set our circadian rhythm, but 8.5 hours would be too fast.

The best way to adapt would be to keep the time you slept and awake the same and use artificial light for night when you want to be awake and heavy curtains for daylight when you want to be asleep, as mentioned in ThePopMachine's answer.

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Now we have two contradictory answers. Some sources to back up each view would be nice. –  jarlemag Apr 18 at 9:43
    
Added link for sources. Also, clarified my answer, so that it is more apparent that our answers don't really contradict. –  Jonathan Apr 20 at 15:13
    
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? –  Superbest Apr 21 at 20:37
    
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? –  Jonathan Apr 21 at 22:20
    
I think the important question is whether or not your circadian clock can adapt to faster day/night cycles. Jet lag is caused by a large shift in the 24-hour cycle, so it's unclear whether the effects would be the same or how long they would last. –  nbogard 2 days ago
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