Consider a planet where a day, that is to say the amount of time a specific (roughly equatorial) point is in sunlight is much longer than on earth. A humanoid race has evolved on this planet. You can assume that days and nights are roughly the same length as each other.

On Earth we have evolved and adapted to a 24 hour day, our sleep cycles traditionally follow the sun.

If a species evolved on a planet with much longer days (for example daylight cycles of a week or more) would the race evolve to stay awake for much longer periods of time or would they sleep during the daytime?

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    $\begingroup$ In fact there are the people who leave on such a planet: the planet is Earth! They live inside polar circles and experience polar nights. They still sleep mostly as usual people, they haven't evolved to sleep half a year. $\endgroup$
    – Ruslan
    Commented Jan 5, 2015 at 20:50
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    $\begingroup$ @HDE226868, Since when is the night the same length as the day? Checking timeanddate.com for my location, March 16 this year will be a 11h 59m 15s day, and March 17 will be 12h 1m 17s. Quito's length of day this year will never go below 12h 6m 16s. Axial tilt's a **** :) $\endgroup$
    – Brian S
    Commented Jan 5, 2015 at 22:38
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    $\begingroup$ @Ruslan But humans didn't evolve for the polar regions. We can't survive in the wild in polar circles without significant warm clothing, for example. Looking at animals that are naturally in these regions would be a more apt comparison. $\endgroup$
    – Tim S.
    Commented Jan 5, 2015 at 23:12
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    $\begingroup$ There is a problem hiding in this question: Could life as we know it even arise on a planet with such long days? It strikes me as likely that the massive temperature gradient on the day/night terminator might result in massive storms, sweeping the planet clean every day, unless the atmosphere is thick enough to properly distribute the temperature, in which case runaway greenhouse effects might very well make it unlivable in any case. (c.f. Venus) $\endgroup$ Commented Jan 6, 2015 at 9:27
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    $\begingroup$ This question is very Earth-centric, in that it assumes the alien life forms will need to sleep in the first place. $\endgroup$ Commented Jan 6, 2015 at 16:59

9 Answers 9


To understand how sleeping cycles would work on a planet with a long sleep cycle, we need to understand why we sleep at night.

Let's put aside the special functions of sleep itself (which has recently been attributed to brain cleaning), and focus on some prominent evolutionary aspects of sleep

  • Sleep helps prey avoid predators

    • Predators tend to have better eyesight than prey. You don't need to have good eyes to hunt grass/trees. It's more efficient to just not move for a significant part of the day than to develop highly sensitive senses.
  • Sleep helps all animals conserve energy

    • Most desert animals are nocturnal to avoid heat, whereas animals in other biomes are diurnal to avoid most predators.
  • Animals tend to alternate sleep cycles with their biggest competitors.

    • Hawks hunt during day and sleep at night, while owls sleep during day and hunt at night.
  • Predators tend to sleep longer than prey because of the high energy density of animal meat

  • Because of the low energy density of plant matter, herbivores sleeping and eating habits are directly proportional to speed.

Let's translate this to a humanoid species on a planet with an arbitrarily long day/night cycle. Though we assume that they have the same basic attributes of normal humans, in reality there would be a high range of answers based on a high number of variables

Because sleeping during the whole night cycle would allow nocturnal predators an arbitrarily long time to find prey, the humanoids probably wouldn't have evolved to sleep the whole night. Because of the lack of a consistent semaphore (sunset or sunrise) and because of it would be imprudent to sleep all night, sleep as a whole may not be a thing at all, however that doesn't rule out the possibility of rest. As far as I can tell, all animals benefit from conserving energy by staying still. Therefore, I think it's safe to say that this humanoid species (as well as all animals on this planet) may not sleep at all, but rather rest.

Sleep is a weird thing. To go entirely unconscious makes little sense except to survival, but to rest makes much more sense. You can't really run away if you can't even tell that your predator is right behind you. In this situation, intermittent decreased conscious rest (perhaps even dolphin-like sleep with half brain sleep at a time) may be much more effective than unconscious rest.

[EDIT] - I wanted to append a few more insights I had today

  • A longer night might mean it gets much colder in the middle of the night than on earth (This is dependent on the density of the atmosphere, temperatures don't change nearly as much during night in the ocean like they do on land). If that's the case, then predators and prey alike may hibernate during the long night. Hibernation is different from sleep. Unlike sleep, hibernation's primary goal is to preserve energy during temperature change, decreasing brain and metabolic functions. Humanoids probably wouldn't dream during hibernation because it costs expensive resources

  • If there's an axial tilt on the hypothetical planet, and the planet revolves around its star with only a few "days" every year, temperature changes would be complex and chaotic near the North and South. While equatorial organisms may find it easier to hibernate due to the predictability of the night, organisms further away from the equator may just choose to always stay awake and tough it out. They would be tough, fat, and slow animals.

  • If the day to night cycle is long enough (maybe around 3 months, or the point when expending energy to change is better than conserving energy and staying the same), animals may treat the night like a season of it's own by shedding coats at sunrise and engorging themselves at sunset. Animals like the arctic fox may change color with the night becoming darker in order to camouflage

  • Depending on the length of night, the sunset and sunrise may each take days to complete. On earth, the sky is lit for about 30 minutes after sunset and before sunrise. This translates to 1/24 of the day being sunless, but light. Just like on earth, this period of time would spur the most activity as animals frantically prepare for the night/day.

  • Some animals (especially birds) may migrate with the day if the night is too long and too harsh. These animals would circumnavigate the globe every "day" on the planet. There could also be animals that migrate with the night due to niche differentiation (the hawk/owl principle). I think the idea of wholly day/night animals would be really interesting.

  • Finally, the reason why human sleep is vital to health has recently been attributed to cleaning. (The purpose for dreaming is still unknown). Because blood doesn't flow freely through the brain, the usual way to remove toxins is absent. The answer to this is to effectively shut down the brain for cleaning every day. Though this (like everything in this answer) is speculation, it may be reasonable to assume that the brain doesn't need to be cleaned if blood is pouring through it (however, blood may age brain cells too quickly due to free radicals -- free floating mitochondria attributed to aging), or some other fluid flowing through the brain which would then be cleaned in an organ similar to the kidneys.

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    $\begingroup$ Then maybe this humans would sleep two times a day - in the middle of the night and the middle of the day. Given that a large planet would have large diffrences in temperature, sunrise and sunset seem like the better times to find food instead of the whole day/night. Depending on the day's length, this might also avoid overly long sleep and ~12-14h of sleep are not that special if you had a long day, even with our day/night cycle. $\endgroup$
    – Sebb
    Commented Jan 6, 2015 at 19:12
  • $\begingroup$ This could be a very likely case, but it's just one possibility on a large spectrum. $\endgroup$ Commented Jan 6, 2015 at 19:24
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    $\begingroup$ When you mention resting as opposed to sleeping: Dolphins would drown if they slept, so they sleep with one-half of their brain at a time, so the other half can keep them afloat. Such a technique could be applied to the long-night case. $\endgroup$ Commented Jan 6, 2015 at 19:57
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    $\begingroup$ @bananafacts Sure, this just came to mind while reading the point about large planets. I think some day-active desert scorpions and/or ants even avoid the noon heat below the sand, this could be a further developed form. Depending on the planet they may even have some kind of winter sleep during midnight or noon. Thinking of such planets, we could be talking about a brown dwarf, maybe in a 2+ star system, but I'm not specialised in this kind of sience. $\endgroup$
    – Sebb
    Commented Jan 6, 2015 at 20:22

Maybe animals would evolve so they don't need to sleep at all. Or like dolphins, half of brain sleeps and other takes care of breathing.

Reason for sleep to evolve - seems that it is needed for synaptic changes.

On your planet, under different evolutionary pressure, rest without sleeping (or sleeping with just half brain) might be better adaptation.


I'd say we would adapt a longer awake/sleep cycle, and the reason for that is simply evolution. That or we would come up with pretty impressive night vision.


option #1 is we evolved from creatures that had no artificial light and no means of really seeing during the dark hours. Our sleep habits were well ingrained into us long before the invent of fire (light during darkness). With this in mind, I think it would be entirely feasible for a species to develop a week long 'hibernation' sleep style where they binge sleep for a week long darkness in preparation for a week long active cycle.

Option #2 is we evolve some mechanism / ability of being awake and active during darkness hours. A 'nightvision' or some other ability to ensure that 'we' (we being our early ancestors) could be active and remain safe during darkness hours.


I sleep during the daytime when I have a chance! And so do many others. Naps are great! (This includes most other animals, dogs, cats, horses, cows, deer, etc).

However, I would expect that reaching a 'week' long day they would have to rest a lot in there. My guess would be they would sleep less (more long naps), while it's day and more while it's night (be awake less). It is also likely they would have more specialized eyes, maybe even two sets, an infrared like snakes or blue like dear and a normal set like ours for daylight. A day set and a night set. Unless they are more like reptiles where not eating for long periods of time isn't a problem.

The likely reason for this would be a planet that is farther along it's path toward tidal locking with it's primary.

The earth used to have 6 hour days a very long time ago. But I think the sun will burn out before make it to 36 hour days (at our current pace).

You also have to worry about warming and cooling of the planet if the faces take that long to move into and out of the sunlight. Possible the equator isn't very hospitable.


I don't quite think people are understanding evolution here. We haven't really "evolved for a day/night cycle of 24 hours". It would be more correct to say that the average person wants to spend about 33% of the time asleep. It's most convenient to this in a "sleep 8, awake 16, repeat" pattern because it allows for regular scheduling of the rest of our day, being awake at predictable times relative to our friends, family, jobs, etc, and most people want that 8 hours of sleep to be at night because we sleep better with sensor deprivation and lack of light helps with this, and finally it's just easier to keep track of your sleep and not ask "wait when should I plan on going to bed again? what day is it? Which part of the rotation am I on?"

There's no reason one couldn't be in a "sleep 6, awake 12, repeat" pattern, or a "sleep 10, awake 20" pattern, but for reasons not at all pertaining to evolution, one would probably not wish to do this. If the earth suddenly magically had daytime periods everywhere that were a month long and then dark periods that were a month long (somehow keeping everything else the same), we'd all learn to adapt. Whether we evolved or not would depend on what sort of mutations came up - but people often talk like evolution has some sort of conscious purpose or strives to achieve a goal. There's nothing that "tries" to even our body's genetic hardcode to the day/night cycle. It's a matter of a random variable, really, if a significant number of people (some of whom marry each other if the genetics are recessive) obtain a mutation that adjusts their sleep/rest needs without negatively affecting some other key part of their life.

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    $\begingroup$ You could argue that our natural cycle (before artificial light) is not 'sleep 8, awake 16, repeat" but rather 7-9 hours split among multiple "sleeps" per night and a daytime nap, there is some evidence that suggests that it was also common historically to have a waking phase during the night between sleeps. $\endgroup$
    – Peteris
    Commented Jan 6, 2015 at 14:50
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    $\begingroup$ By saying "we haven't evolved for 24 hours" and "there's no reason one couldn't...", do you intend to deny the existence of 24-hour-ish endogenous biological processes in humans ("circadian rhythms")? I don't deny that humans can survive an 18 hour pattern of sleep 6/wake 12 as you describe, but I think we're somewhat better adapted for 24 hour cycles, in ways that aren't so simple as an immediate response to whatever light we observe. $\endgroup$ Commented Jan 6, 2015 at 18:43
  • $\begingroup$ In Europe (as recently as baroque) it was called 1st sleep, "watch", 2nd sleep. I saw an etching that had: people playing games by candle light, and someone tending to an elderly person who was still in bed. . Winter, I guess. $\endgroup$ Commented Jan 10, 2015 at 7:11
  • $\begingroup$ Circadian rhythms are real biological cycles that vary on a roughly 24-hour period, corresponding with the length of a day. The "33% of the time asleep" model is in no way more correct - it would be extremely unpleasant to stay awake for four full days and then sleep for two. 24-hour sleep cycles aren't sociological, they're biological. $\endgroup$ Commented Aug 29, 2019 at 17:15

The pattern of sleep would probably be dependent on the metabolism of the sleepers. An animal with a very slow metabolism that could survive through the night without needing to go into hibernation would probably have its leisurely meal for the day, and then sleep through the long night.

With a human like metabolism, this might not be possible. If the night lasts as long as a week on Earth, sleeping through the whole thing would be detrimental to health. Water loss and food loss during sleep are simply to high to allow this. It's likely that something with a human-like metabolism would wake up intermittently throughout the night to eat and drink, probably from food collected during the long day if the creature had evolved to be diurnal.

A human-like creature might also sleep, or at least rest, intermittently throughout the day to allow digestion and healing to take place. For a tool-user, these could be long periods for performing less strength-intensive work. This pattern of active and restful periods without sleeping is found in sharks on Earth.

Night time activity requires the evolution of traits that are not without their costs. Eyes which evolve to see well at night generally don't see as well during the day, and vice-versa. While it's possible that creatures living on a planet with long nights might evolve better night vision, there would still be strong evolutionary pressure for specialization.

Lastly, if the day/night cycles were on the scale of Earth's months or years, it's likely that animals would follow a pattern of hibernation, with metabolic activity dropping off sharply during the long night while the animal holes up in a burrow to sleep until the next day.


It is still not very clear to science what sleep is all good for, and why it has evolved, but since it so ubiquitous, it is only fair to assume that it is absolutely necessary for somewhat higher brain functionality (though research indicates that even simple flies have a state that could be described as sleeping).

Depending on what research you follow/believe, sleep seems to serve some purposes:

  • Save energy for the part of the day where you can not do much useful (humans are bad for hunting in the jungle at night)
  • Repair chemical/neural "damage" done during the use phase of the brain
  • Reinforce learned knowledge (It is somewhat controversial how it is done, but people seem to agree that it is being done)

So for the energy save part, looking at mammals that rest or sleep through a winter, we can safely assume that from the energy perspective adapting to a week or even a month sleep can work for humanoids.

If you ever had the opportunity to stay awake much longer than 24 hours (say 50 or 60) you notice that your brain pretty much malfunctions (or you don't notice, as part of that malfunction). This gets much better when you do some power naps. I am confident that evolution could work its way through all this getting tired and accumulating bad stuff in your brain that gets cleaned up later. Enough to stay awake for a week, not so much for a month though.

The learning part though is a tough one since you would accumulate a week of stuff in your short term memory which will then get sorted over one week of sleep into the long term memory. This makes you require quite some short term memory and has quite some long turn around time for efficient learning. So either the species isn't that efficient at learning, or it has some shorter sleep cycle that is being used for this.

This would be in contrast though to the inability to properly cope with predators that hunt at day and would find you lying around and sleeping.

My idea for this would be a highly segmented brain and a very social structure.

Just like with dolphins, the brain will sleep partially, but not only two parts, but maybe four. During that time you are basically in some kind of "autopilot" mode, being able to do simple things like walking, but not more complex stuff. Additionally the social abilities of the humanoids organize themselves around these states of mind, so that if e.g. a group of them are traveling some are in autopilot and others take care of them.

Given that the social abilities of humans are a great part of their evolution, I think this would be a pretty likely scenario for humanoids to evolve for a week or month of day. With increasing intelligence and use of tools, they would probably evolve similarly for the night.


I think it would depend on the species.

For an animal to sleep for an entire week, it means that its body has enough spare fat and water to support it for that week (and some extra to begin feeding the next day).

Only the biggest animals could support that; others would have to adapt to being active both at day at night. Of course, that does not mean that they would behave equally in both situations; vg:

  • A "rabbit" would travel and mate at day, and, when the night is closer, create a new burrow near a supply of vegetables and water.

  • A "squirrel" would pack during the day its nest with fruits. It would prefer fruits with high water content.

  • Likewise, ruminants would graze during the day and spent the night "reprocessing" half eaten grass.

  • $\begingroup$ Hedgehogs can sleep for three to four months, surely evolution could adapt to have a one week sleep/awake cycle for animals of similar size. $\endgroup$
    – PlasmaHH
    Commented Jan 6, 2015 at 18:41

It is hard to answer the question because it makes a large, incorrect, assumption. The fact of the matter is, humans are not evolved to sleep for 8 hours straight.

The 16 wake/8 sleep cycle is a recent development. We only started to do it after the invention of the light bulb, and even then it only really started to pick up when labour moved out of farms and into factories.

Before that siestas (mid-afternoon naps) and waking up for a midnight snack was the common, expected, routine.

Even ignoring that, there are so many different sleep patterns amongst animals on earth, that you can do whatever sounds cool in your world and it probably won't seem out of place.

I mean, sleep patterns range from cats, who are in a constant state of fluctuating from awake to asleep, to animals that hibernate for months at a time. Marine animals have evolved to sleep half their body at one time while the other half is awake. When one side is fully rested, the other side falls asleep and the first side wakes up. I'm sure there are some sleep cycles that are truly utterly bizarre.


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