On habitable planets, rotational period can be assumed to vary significantly. What are the limits of human adaptation to these different conditions while maintaining a normal sleep schedule? The humans have to stay awake for a clear majority of the day, and take the clear majority of their sleep during the night.

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Zakoo is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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    $\begingroup$ Just out of curiosity, why is the requirement that they take the majority of their sleep during the night? I myself do multiphasic sleep, meaning I do not do a single sleep per day, but usually two to four shorter, averaging around 2.5, but my total cycle is still about 24 hours. And while I do encounter doctors who think this is a bad idea, most of those I've talked to think it's perfectly reasonable, as long as I get enough total sleep. $\endgroup$ Jun 21 at 16:30
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    $\begingroup$ Have you heard of Iceland? people live there you know, have done for centuries, they're just like you and me, no physiological changes have resulted, no daylight in winter and no night in summer, in short there is no limit and no 'adaption' is needed, does that answer your question. $\endgroup$
    – Pelinore
    Jun 21 at 16:36
  • $\begingroup$ @InTheAbsenceOfFear The people are allowed to sleep over multiple sessions if they want to, and take naps during the day, but on average they still have to sleep significantly more at night then during the day. $\endgroup$
    – Zakoo
    Jun 21 at 18:12
  • $\begingroup$ @Pelinore Yep, but despite this, to my knowledge, they still stick to a 24-hour sleep cycle, even if it is always day or always night. $\endgroup$
    – Zakoo
    Jun 21 at 18:18
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    $\begingroup$ @Pelinore Opposite scenario would be people living on the ISS, where a "day" is only an hour and a half. They still keep a 24 hour sleep/wake cycle up there. $\endgroup$ Jun 22 at 14:21

6 Answers 6


Studies done on humans removed from the synchronizing effect of solar light exposure show that the circadian rhythm of sleep tends to re-synch around 36 hours.

It seems therefore that the human body can tune itself around that duration, even though it has to be seen how the solar exposure or lack thereof will influence it: people living at very high latitudes, experiencing long night/days during the year, do not seem to diverge from the 24 hours cycle. However it must be noted that there are other forcing factors which might influence the sleep/wake cycle.

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    $\begingroup$ I should point out that there are no studies which focus on developing humans being switched to a different day/night cycle, just full grown adults, because performing that kind of long term study on children is both impractical and horribly unethical. $\endgroup$
    – Halfthawed
    Jun 21 at 17:45
  • $\begingroup$ @Halfthawed not that horrible: there are kids living close to the nordcape in norway, and some of them could arguably move to Oslo within their juvenile time and vice versa, so their natural adaption from midsummernights to normal day night cycles could be observed. We also do have knowledge that the long nights of winter can result in a lack of hormones in kids, which can be cured by a dose of 10 minutes high UV light - which was regularly done in soviet settlements in the polar regions. $\endgroup$
    – Trish
    Jun 22 at 5:44
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    $\begingroup$ @Trish: I would like to note that there is a difference between high latitudes (varying day/night proportion in a 24h cycle) and varying cycle length (for example to 36h, as mentioned in this answer). They are so different that I am really unsure that any experiment with the former can give us any information on reactions to the latter. $\endgroup$ Jun 22 at 12:06
  • $\begingroup$ In midsommer, there is no cycle ad the nord cape, while in Oslo days are pretty normal-ish with just about 3/4 daylight. $\endgroup$
    – Trish
    Jun 22 at 12:24

For many year the USN submarines used a 18 hour day, 6 hours on watch, 6 hours of sleep and the remaining 6 hours for meals, additional work, and free time.

  • $\begingroup$ Reference/Link please? $\endgroup$
    – BCLC
    Jun 23 at 4:31

How long are we talking about?

Evolution is your friend. Plop a bunch of humans onto a planet with a 100-hour day and a 100-hour night and wait 50,000 years and you'll be surprised what they evolve to do. Given enough time, any modification you wish is within suspension-of-disbelief.

But the shorter the time span, the less believable it is. Plop those same humans down and wait a month and what you should expect to find is some creative shutters that block every possible photon during the day while our intrepid humans happily sleep 7-10 hours a day.

So, the real question is, "given that I want humanity to conform to [insert very specific day/night conditions here], what would be a believable amount of time for humanity to achieve this conformance?"

  • $\begingroup$ "what they evolve to do" - sure, if they live as hunter-gatherer tribes for most of the time. But if they have a technological civilization, then the environment (unless extremely lethal) will likely play a negligible role in sexual selection and survival. $\endgroup$
    – vsz
    Jun 22 at 4:16
  • $\begingroup$ please add multiple 0s to your years to call it evolution. The scale you use is adaption and not a genetic alteration. $\endgroup$
    – Trish
    Jun 22 at 5:47
  • $\begingroup$ either nature will come up with new way to increase capacity/durability of the body (better metabolic system, better ATP storage/conversion etc) or maybe by then we can let half of our brain sleep alternating every 8-10 hrs during day before full rest during night $\endgroup$ Jun 22 at 8:25
  • $\begingroup$ @Trish Fruit flies have speciated in 35 generations. Humans show many genetic adaptations to local environments, many showing up in a few thousand years; one very visible adaptation is skin color matching UV exposure. Genetic mutation is continuous, and when there's dramatic changes in environment, evolution will start causing changes in a few generations. $\endgroup$
    – prosfilaes
    Jun 22 at 19:10
  • $\begingroup$ @vsz I re-read the OP's question and can't find any mention of sexual selection or survival - only the question of adapting the sleep cycle. $\endgroup$
    – JBH
    Jun 24 at 9:01

As others have pointed out, humans are pretty adaptable in this regard, so the limits will be set by factors other than human biology as such.

The lower limit will be on the order of a couple of hours; much less, and the integrity of the planet with regard to centrifugal force comes into question.

The upper limit will be on the order of a couple of years; much more, and it's impossible to store food overnight, and everyone will have starved to death by the time morning comes and agriculture is possible again.


Short Answer:

I don't know.

Long Answer:

Nobody knows exactly. But several other answers have mentioned various factors to consider. And here is something else to consider.

If future humans build space habitats they will have control over the periods of light and darek in any "outdoors" parts of the space habitats, and of course they will have artifical lighting for inside their homes and workplaces, etc. So they will have total control over the lengths of days and nights in those space habitats.

And if humans build giant arcologies on Earth or other planets, most or all the interiors of those archologies will be illuminated by artificial lighting, so the light levels outdoors will only matter to those who go outside on rare occassions.

And the same goes for underground cities on Earth or other planets.

But if humans settle on naturally habitable worlds, other planets or moons, or terraform other planets or moons to be habitable for humans, and if those humans decide to live as much outdoors as much of the time as modern humans do, the natural cycles of light and darkness will be important to the colonists whenever they are outdoors, and going outdoors will be something people do several times a day.

Thus the colonista - if they choose not to spend all of their time indoors in giant archologies - will have to adjust their cycles of sleeping and waking to the natural cycles on the planet.

And most of the answers so far have discussed how well humans could adjust to day night cycles of different lengths than Earth's.

But suppoe that human settlers on an alien planet aren't advanced enough to have food synthsizers to produce food out of raw materials and instead have to grow food outdoors. If the humans can't find any native plants they can eat, they will have to plant crops of Earth plants.

And those Earth plants will have to be capable of surviving and even thriving on the alien planet, which will mean that the conditions on the settled parts of the planet will have to be similar enough to the conditions on the densely populated parts of Earth for the plants to thrive.

So finding out the limits of day and night lengths that major Earth crops can tolerate would be important for answering the question.

And of course humans need the right atmosphere to breathe. And of course on Earth the oxygen which he need was produced by plants using photosynthesis. If the planet doesn't have native plants, or plants imported from Earth, to produce oxygen humans won't be able to breathe on the planet and will have to live totally indoors in arcologies with totally sealed environments like moon bases would have. And if they do that they will have artificial lighting indoors and will replicate the day/night cycle of Earth, so the question would be meaningless.

Of course it took hundreds of millions or billions of years for photosythetic plants to produce enough oxygen that it accumulated in Earth's atmosphere faster than it combined with minerals and left the atmosphere. If humans bring Earth plants to a planet without oxygen producing plant life, it may take many millions of years longer to build up a breathable atmosphere than the needs of your story allow.

So I guess that the humans in your story will explore, crashland, or settle on a planet which has a naturally oxygen rich atmosphere.

And maybe the humans would keep an Earth like approximatley 24 hours cycle of light and dark inside their homes, and only go outside when the outside light period coincides with their waking period of light inside. Depending on the mathematical relationships between the two periods, the times when the interior human day and the exterior planetary day can be as common or as rare, as long or as short, as fits the story.

See also the discussion of the possible length of day on a habitable planet in Habitable Planets for man, 1964, Stephen H. Dole, pages 58-61. Dole worried that plants might die in the sunless nights if they were too long.


And also see discussions whether planets which are tidally locked to their stars, having their rotation periods slowed so that one side always faces the star and the other side always faces away from the stars, can be habitable. The day side of the planet might get too hot, and all the water and even air might freeze solid on the bitterly cold night side. But possibly the water and atmosphere might carry enough heat from the Eternal day side to the eternal night side to keep both sides at temperatures suitable for life. But possibly an atmosphere thick enough to carry enough heat from the day side might be so dense it would block starlight from reaching the surface of the planet, making photosynthesis and oxygen production impossible.

Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri[84] (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal locking. One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side,...

This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 millibars (0.10 atm), for the star's heat to be effectively carried to the night side.[85] This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[86]


But could alien plants survive in eternal day?

I'm sure that there has ben some research in subjecting Earth plants to artifical light for periods longer than normal daylight, and possibly even to constant 24 hours a day light. If that research show that it is easy for Earth plants, adapted to a day-night cycle, to flourish with 24 hours a day illumination, then alien plants adaped to their world should do fine with 24 hours a day light. So that is something to look up.

And of course if alien plants could flourish in eternal day, other alien plants should be able to flourish during alien days and nights which last for several Earth hours, Earth days, Earth weeks, Earth months, Earth years, Earth decades, Earth centuries, Earth millennia, etc., so long as the temperatures don't get too hot or too cold.

The people would just have to stay inside with artificial cycles of light and dark, and only go outside when their artifical light and waking time coincided with the natural day outside on the planet.


I don't have hard scientific facts, but these:

  • The brain needs dreaming for "garbage collecting", typically done when sleeping, so there needs to be some balance between sleeping and being awake.
  • The eyes need a darkness phase to recover
  • Skin needs light for vitamin D production
  • Sunny days can reduce the amount of sleep needed, while the opposite may be true, too
  • On short term (a few days) one can reduce sleep significantly, but eventually the brain will switch to "emergency shutdown mode" (meaning you can fall asleep while standing, or even while driving a car)
  • Digestion also needs periods of rest; they say sleeping actually helps loosing weight
  • Heart and muscles probably also need periods of rest.

So (as others said already) the day and night cycle influences the sleep/wake cycle, but the limits are not very flexible. I also think there are no real long-term experiments exploring the limits

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U. Windl is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • 1
    $\begingroup$ Hi U, welcome to Worldbuilding! The question already states how "a normal sleep schedule" is maintained, so some of your points are irrelevant to the question. That humans require (natural) light and darkness for some processes is true, but the question is how this limits the adaptability of humans in that regard. $\endgroup$
    – Joachim
    Jun 22 at 7:49
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    $\begingroup$ I tried to point out that "nobody knows" (due to lack of trying, because of the reasons given in my answer). $\endgroup$
    – U. Windl
    Jun 22 at 8:10
  • $\begingroup$ Not quite true. The experiments have indeed been done as mentioned in two of the above answers (though I personally would like to have seen a citation or two, but was familiar with them anyhow). $\endgroup$ Jun 22 at 8:42
  • $\begingroup$ I mean: Did anybody ever try an 48h wake / 48h sleep over a longer time for example? I read about the 18h day, but then is 18h day the only possible solution? And what does the 36h resync actually say? $\endgroup$
    – U. Windl
    Jun 22 at 10:14

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