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We have 24 hour schedules because the earth rotates about once every 24 hours. But what if humans were on a planet that didn't have 24 hour days? Would their bodies adapt to stay awake for far longer or shorter periods of time, then likely also sleeping longer or shorter, or would their bodies still gravitate towards a 24 hour cycle?

I assume people living near the poles would likely have the best idea of this, or perhaps those who have lived for an extended time in a space station. I would think that, though our bodies often adapt to their surroundings, that a roughly 24 hour cycle is healthiest for the body. This is because the body seems to do well when it gets enough sleep, but also shouldn't sleep too much, as one needs to eat and (optimally) work to stay in good health. Sleeping is often recommended to be 8 - 10 hours, but is that because of our 24 hour schedule?

So, if a human lived in an environment where daylight was too long/short to go by, or no star was available to go by, what would be the optimal time schedule for them? Would they still be inclined to go by a 24 hour time, even if they may not be familiar with such a schedule prior.

This is primarily to determine: if people lived on planets where the daylight schedule couldn't be used as days, or lived in space with no star to go by, what time schedule would they gravitate towards (likely for health reasons)?

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    $\begingroup$ I had heard somewhere that humans naturally have a ~23 hour day internal clock since that was the day length when are brain was evolving $\endgroup$ – TruthOf42 Jun 24 '17 at 4:32
  • $\begingroup$ skeptics.stackexchange.com/questions/37366/… $\endgroup$ – apaul Jun 24 '17 at 4:59
  • $\begingroup$ cabinetmagazine.org/issues/30/foer.php? I originally read that in a NatGeo issue from nearer that date. Proper answer is forthcoming, but here's the gist of it: natural tendency in the experiment was for 36 hours of wakefulness and 12..14 hours sleep. Their responses and cognition were not measured in deviations from that, so we don't know if it could properly be called a limit or an optimum. $\endgroup$ – can-ned_food Feb 8 '18 at 6:21
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I have a little personal experience with this sort of thing, one of my first jobs was cooking short order. My shifts tended to flip between 6pm to 6am or vice versa...

Honestly it takes a good while to adjust and the first few flips can be painfully jarring, but after an initial adjustment period of a few months... Well, you'd be surprised what you can get used to.

After a point you can separate from the typical circadian rhythm. Having a windowless room to crash/sleep in will help tremendously. And yes, developing a cycle of greater than 24 hours is entirely possible.

If you don't want to take my word for it, The National Center for Biotechnology Information made similar observations:

At Arctic and Antarctic latitudes, personnel are deprived of natural sunlight in winter and have continuous daylight in summer: light of sufficient intensity and suitable spectral composition is the main factor that maintains the 24-h period of human circadian rhythms ... A few people desynchronize from the 24-h day (free-run) and show their intrinsic circadian period, usually >24 h.


To more directly address the question...

After taking the time to adjust​, people can become pretty flexible. At a point, you sleep when you're tired and eat when hungry. Artificial light control can help with adjustment.

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That's hard to say, but I know of studies that point that trying to change the cycle usually leads to health problems. If you have your population living long enough under determined circumstance it's possible they will adapt, but until them be ready for fatigue, disorientation, insomnia, and possibly mental disorders.

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Humans are living beings. Living beings have been developing (so far, to the extent of our knowledge) on a planet with a day of 24 hours. Slowing down of the day is way too low to be noticed during few generation of most if not all the species, so we can neglect it.

Thus, based on this durable influx of the environment, the 24 hours cycle is hard coded in our bodies. Therefore attempting to force it outside its range is very likely to mess things up, like the pop up on the registry editor of most home OS says

"change this configuration at your own risk"

Adapting to a new day cycle, for example because we live on another planet, will pose a tremendous challenges on our body. As individuals and specie we will probably be defeated (look at how many issues can arise in the life of shift workers "just" beacuse they don't sleep at night) by the challenge and have to stitch to our 24 hours biologic clock.

If the evolution will be able to select a different path, instead, is a question I dare to answer with "Yes, probably" (the "probably" being reserved for borderline situations like a tidally locked earth or a Mercurian situation of extremely long day.

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    $\begingroup$ skeptics.stackexchange.com/questions/37366/… That's not necessarily accurate. $\endgroup$ – apaul Jun 24 '17 at 5:01
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    $\begingroup$ I can also tell you from personal experience, being a shift worker for a lot of years, that humans may be more flexible than we give them credit for. $\endgroup$ – apaul Jun 24 '17 at 5:03

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