I realise this question is rather broad, so I'll do my best to narrow down what I'm looking for as much as possible.

The big idea

The full daylight cycle on this hypothetical planet is consistent in duration, but it is made up of two smaller cycles that are not. Essentially, this is how it goes:

  • First Day (~12 hours)
  • First Night (~4 hours)
  • Second Day (~12 hours)
  • Second Night (~16 hours)

Following this the cycle loops around to the first day, and so on.


  • The main question: what would a human population's sleeping schedule look like? Would they sleep once at the beginning of the second night, wake up in the middle of it, and sleep again near the end? (this is my current hypothesis)
  • In a similar vein, if a significant portion of this human society were to be active in the middle of the night, would their activity be very different to what it'd be in broad daylight?
  • Would this affect plant life in any way? (I assume not much, but just asking to be sure)
  • What about animal behavior with things like sleeping and hunting patterns?

Some extra clarifications

  • Assume the human population lives in some sort of medieval society
  • Assume an earth-like planet for vegetation, climate, etc. (say the northern USA if you need a region)
  • Assume a seasonal cycle like what you'd see in north america (clear separation between summer/winter, and consistent/comparable length of seasons/year)
  • Assume that this cycle applies to a particular region and not necessarily the whole planet. I'm not sure how the day/night cycle would differ with seasons and lattitudes (I assume it would change at least slightly), but if you happen to have an idea I'd also like to know.
  • I'm not even sure if such a cycle is possible, neither of what sort of convoluted astral combination you'd have to use (maybe with two suns or something?) but for the purposes of this question, assume it exists in at least one region.
  • $\begingroup$ I most places of this world, the duration of day time and night time varies quite a bit during the year. For example, here in Bucharest it varies from 15 h day + 9 h night in June to 9 h day + 15 h night in December. In Saint Petersburg night time varies from 19 hours in December to 6 hours in June, during which month there it is never full dark at night: and yet I've never heard that they have trouble sleeping in the northern capital of Russia. $\endgroup$
    – AlexP
    Commented Aug 13, 2021 at 15:54
  • $\begingroup$ I'm imagining a similar dynamic that could happen with your world being a moon around a gas giant. Your moon spins twice per orbital period, perfectly lined up so that the night will align with the time where the sun is eclipsed by the planet. The difference would be that this would only apply to one region of the planet at a time, and that region would change as the (planet's) year progresses. $\endgroup$ Commented Aug 13, 2021 at 16:07
  • 1
    $\begingroup$ Read up about biphasic sleep (en.wikipedia.org/wiki/Biphasic_and_polyphasic_sleep). TL;DR many think that the pattern you propose is actually what pre-industrial humans did $\endgroup$
    – Guest
    Commented Aug 13, 2021 at 17:47
  • $\begingroup$ Well, one can't really just address how humans would adapt until they knew how the flora and fauna adapt to it, as that is a pretty big factor in medieval society. $\endgroup$
    – Firestryke
    Commented Aug 15, 2021 at 19:54

3 Answers 3


One short and one long sleep cycle.

Diurnal animals have evolved to sleep when it's dark and be active during the day. This situation will be no different. Instead of having one long sleep cycle every 24 hours, most animals in this world will naturally tend to have two sleep cycles every 48 hours, one short and one long. It's basically the schedule of a person who regularly takes a nap in the middle of the day, but with even more biological impetus to do so. Animals aren't tied to a 24-hour clock, they're tied to the natural cycles of their environment - there's no reason why humans should become active in the middle of the night just because the clock says they've been asleep too long. Rather, the "right" amount of sleep will depend on the day/night cycle. Humans and other diurnal animals will tend to sleep through periods of darkness and be awake during periods of light.

  • $\begingroup$ For me, this would be most enjoyable as a reader because then the author can discuss culturally interesting things, like reverence for the short sleep, but it's often neglected. $\endgroup$
    – user458
    Commented Sep 14, 2021 at 16:32

Actually, I think this may work just fine with Earth biology.

Circadian clock bits

Earth organisms have an internal, self-timing circadian clock set to just over 24h - if kept in constant darkness. As soon as there is any light (or other external cues), the clock is reset. This means that the circadian clock will not be thrown off by the different nights' duration - all you need to track is "get drowsy once you've been up for ~12h" and you'll be fine.

Human behaviour bits

Assuming our ratio of 16h awake/8h asleep still applies, there could be two ways to form a rhythm:

  • A long sleep (about 16h), overlapping with the long night, then stay awake and active through the day-short night-day phase.
  • Take a nap somewhere around the time of short night (could be a bit later - our waking hours are often shifted later than the daylight, such that we wake up after dawn and go to bed after sunset) and sleep a longer sleep (maybe 12 hours) during the long sleep phase, again probably shifted towards the latter phase of the night.

Being awake for 28h is not great, at least for Earth-adapted organisms, so I think the second pattern is more likely; but both might exist.

Animal behaviour bit

Animals already have very different sleep patterns based on their ecological niche, predation techniques, availability of food, etc. What would make more of a difference is that the short night is likely to become a bit of a unique niche, because in 4h between sunset and sunrise the sky would probably never get entirely dark. Crepuscular animals would have a much bigger chunk of the clock, and perhaps there would be specialised niches for the short nights, for the twilight of the long night, and for the dawn of the long night. Some of this will depend on whether your two days are entirely identical or if there are some unique features (e.g. different light spectra because the light comes from different suns).


The biology and evolutionary aspect of sleep, in a species

Bear in mind your native life would have the full history of evolution since "day" #1, based on this kind of irregular cycle. Perhaps gradually changing over periods of hundreds of millions of years.

So the non dramatic answer will be based on the idea that there never was pressure to any earthly cycle. There may not be sleep as we know it. You certainly cant start by treating it as if this is earth with a change now made - these people never had earth, nor did their primordial ancestors. Ever. Right back to the equivalent of single cell precursors to amphibians.

Its a bit more complex since we don't actually understand the function of sleep. We have some ideas - some activities work best with different light levels, some brain and body restoration and consolidation benefits from an altered state. We dont really understand sleep, though, or understand why it's so pervasive on earth.

It may be universally needed, or one optimal lifeform strategy. But it could be like front or rear light sensitive cells in the eyes - all mammals have eyes with the light detecting cells at the back, and that's just how we started and keep going over eons. But ots not how all life has to be. Octopi have more efficient eyes, organised the other way round. Parallel/convergent evolution but with one difference retained.

Sleep is like that too. Some animals sleep half the brain at a time, so there is always some awareness of whats going on. Parallel or convergent evolution.

So your native life may need sleep. Or not. If they do, they may need it as we are used to it, or not. Same form or different form. But either way, they will be biologically adapted to it.

By contrast, if any beings originated on another world, they will bri lng their biology with them, and have to adapt. Over time they may adapt well. But like proto-whales going back to the ocean, theyll always carry their native world and its adaptations, with them.

What this means

You dont say if these "humans" are native and just "conveniently human-like", or actual earthly humans whose ancestors came here. Thats crucial, it affects their biology.

If they are natives, biologically, then my guess will be that no regular sleep cycle exists. They will have biology that does its repair and restoration in a different way. They may be more or less active in daylight, or do different activities, or rest brain parts on some cycle, but they wont try to overlay an irregular cycle with artificial periods of rest and activity, to make a regular one. Why would they? They have literally, hundreds of millions of years of evolution as animals, in a place where irregular cycles are the norm, and thats what they will be used to, take as natural, be adapted totally to.

If not native, they'll act like any humans do in irregular shifts and light times, and try to impose a regular 24 hour cycle on it.


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