For this scenario, imagine your classical habitable tidally locked planet: hot desert on one side, frigid wasteland on the other, and the only thing that is good is in the twilight zone.

Now, assume that the planet evolves life. How would life on the planet sleep?

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    $\begingroup$ There are places on Earth near the poles where it's light/dark for months at a time, and life seems to get along pretty well. Humans, for example, have lived inside the Arctic circle for (at least) tens of thousands of years. $\endgroup$
    – user535733
    Commented Mar 7, 2022 at 14:58
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    $\begingroup$ Have you considered that not all forms of life would feature sleep? Alien life that evolved on an alien planet would have a biology that is, well, alien. $\endgroup$ Commented Mar 7, 2022 at 17:45
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    $\begingroup$ GrandmasterB makes a solid argument. No need to overthink things. $\endgroup$
    – Joe Smith
    Commented Mar 7, 2022 at 21:29
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    $\begingroup$ It would sleep like Garfield... whenever it wants to. $\endgroup$
    – Mentalist
    Commented Mar 8, 2022 at 6:34

7 Answers 7


Different forms of sleep exist - there's a great radiolab episode about this, link below.


So, some main things. First of all, sleep isn't dependent on a day night cycle, it just takes advantage of it. Other survival concerns can take precedent over an animal's sleep cycle. Predation is a big one, but animals like dolphins also need to worry about problems like drowning. The solution that many animals seem to have embraced is partial sleep, where the brain will power down some sections while keeping others running. The majority of animals do this, often literally sleeping with one eye open so that, if a predator does show up, that portion of the brain can sound the alarm, wake up the rest of the brain and find a way to stay alive. Fish sleep while a part of their nervous system keeps them swimming, making sure that they can still breath and helping them to avoid predators. Sleep as humans and many other mammals experience it, with the whole body going entirely unconscious, turns out to be the exception rather than the rule.

So, for life evolving on a planet with no real day/night cycle, sleep in phases seems to be the most obvious direction to go in. There are many ways this could work, but one method would be bodies with redundancies. Bicameral brains, working in shifts where one half rests while the other functions, seems like a clear option. Decentralized intelligence, where the nervous system is spread out across the body and different parts can get away with micro-sleep while the rest of the body compensates, is another.

Social and Eusocial behavior also adapts for this. Herd or hive mentality could have some individuals in a group sleep while others protect them, switching off as need be, or potentially not at all. For smaller, insect-like creatures, it wouldn't be unthinkable to imagine drones that never sleep protecting a hive until they die of exhaustion while the queen gets the sleep cycles she needs to keep up the hive and produce more temporary drones.

It is most likely that, to an outside observer, animals on this planet would not appear to sleep. Many would have adapted to give the impression, especially to would be predators, that they are always ready, always awake and aware. However, close attention would show this for a ruse, and probably various predators and parasites would have evolved to take advantage of whatever patterns there are to the partial sleep that these creatures would use.

But nature is full of exceptions. There is an oppurtunity here for something really fun - you know how many birds have evolved mating signals based around intentional disadvantages, like bright coloration or big heavy tails? What if there was a creature - probably some kind of megafauna - that broadcast its fitness to potential mates by going into full blown REM sleep? Like, look at me, I'm so amazing, I can literally stop moving for hours at a time in direct sunlight and nobody can kill me, don't you want my genes? The one doing the selecting would then nestle down and fall asleep next to the individual they found, and some kind of pheromone signalling system would kick in so that their sleep cycles synchronize and they both wake up at the same time, and that's how they select their mates.

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    $\begingroup$ you just invented snorlax $\endgroup$ Commented Mar 8, 2022 at 14:48

Even for a tidally-locked body, libration would occur, in effect, the body would rock back and forth, slightly changing the direction toward which the sunlight side of the planet faces on a regular cycle.

This would cause regular periods of light and darkness or twilight around the twilight zone. It is entirely possible that life forms would evolve to take advantage of their preferred light levels, and might sleep through the other part of the libration cycle.

It is also entirely possible that if sleep is necessary for the life forms on this world, they might synchronize to some other regular cycle, whether astronomical or local, or they might have an entirely arbitrary sleep-wake cycle.

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    $\begingroup$ +1 - And I would use as an example whatever animals in the poles do, as those are the closest to "perma-day" and "perma-dark" places we can draw conclusions from. Or the little that's known from the bottom of the oceans. $\endgroup$
    – Stivsko
    Commented Mar 7, 2022 at 13:15

Sleep in Short Periods

Modern humans sleep for a single period during the night and wake during the day. Some cultures incorporate a nap in the middle of the day. Historically This is because (a) we have bad night vision and (b) have no predators.

For many animals this is not the case. Some animals are nocturnal. They sleep during the day and are active at night. Some animals are crepuscular. They sleep during the day and night and are active at dusk. Some animals sleep in shifts throughout the night and day while the others keep a look out. Some animals never fully sleep. They put half their brain into a rest mode and then swap over after a while. Some animals live in caves and never see the light of day. Some animals have burrows where they hide as well as sleep. Some animals just sleep on their feet standing in big herds in the open.

All of these animals have different sleep schedules. The only pattern is that there is no pattern. It is believable that Earth-like animals in a permanent twilight would still sleep.

The main difference is there can no longer be any coordination between predators and prey. For example many predators hunt by sight, and potential prey animals get an advantage by being active at night.

In a world with day and night and rabbits and cats, the rabbits sleep for one long period in their burrows during the night, and are active during the day when there are fewer cats around.

In a twilight world with only rabbits and cats, I predict the rabbits sleep in shorter bursts. When the herd spots a cat they all flee underground and sleep for an hour. Then they come out when the danger has passed.


They would have daily migrations

In your twilight zone they likely would separate out the day and night by simply finding a shady spot behind a mountain or in a valley, and move to the lit areas during their active period.

What is a day?

Consider that if we follow the current theory of how our life evolved, the idea of a day was a late development. Your planet's first life may have never even known the sun or the surface, and instead began by getting their energy from radioactive decay like these Desulfotomaculum bacteria on earth do, two miles under ground.

Even though your planet does not have a clearly defined day and night, there will always be some subtle variations in the surface climate which a living organism can take advantage of. It could be some eccentricity in the orbit, or it may be a pair of satellites creating a wobble, or it may be the subtle resonant librations of the planet. But in the end, life is not going to do well in a perfectly static environment, because that environment will quickly devolve through entropy. Fortunately, the universe is messy and doesn't really make perfectly static systems.

It's probably not helpful to compare the concepts of "sleep" as we know it on earth to your life forms, because everything here has had the benefit of a very clearly defined diurnal cycle. We work the way we do because we grew up where we did. The metabolisms in the higher life on your planet would not recognize periods similar to our own, but instead, there would be some other cyclic energy flow that would trigger an active period, and a rest period—a circadian clock set by the planet. It would be reasonable to assume your life began just feeding off radioactivity deep in the planet's crust at first, and as it got pushed higher, it found some sort of energy cycle that it could leverage to drive higher metabolic rates in small bursts. Let's assume for now it was some seasonal change caused by eccentricity in the orbit. It may also be a much dimmer star than our sun, so your goldilocks zone is much closer. It is even possible to have your entire orbit happen in one earth day while keeping an earth-like climate. So you can imagine any arbitrary cycle, and any arbitrary cause for it; but what your life will need is to have these cycles of higher and lower energy density, and a more rapid energy cycle will allow your mechanisms of evolution to work more quickly on successful adaptations. At any rate, your lowest life forms need to locate this certain cycle to reach up for the surface. That will promote the more advanced forms, allowing multicellular life which can share resources and coordinate energy spending.

Once these median forms find their way to the surface, they need to come up with some form of locomotion. Your planet's energy cycles need to have a large enough variation for them to store energy for bursts of activity. On earth, this likely happened in the seas, because water was so easy to push yourself through with very little energy. The clusters of multiple cells could all push together, and with a small amount of effort, they were into a new area of fresh energy.

Your life soon gets good enough at having its cells work together, that it becomes the first energy hunter. It figures this out by coming across some sort of sensory input, where a source of energy triggers something in one cell. On earth, maybe one of the first ways we found energy was by cells that reacted to sunlight: rudimentary eyes. It can determine where energy might be around it before it touches it; it no longer has to wait for energy to be delivered by the planet's natural cycles.

This upper-middle organism now has the tools to control its own fate. It can spend energy in larger bursts using some of the cells (we call them muscles), it can deliver energy to some cells that are good at locating energy (sensory cells and nerves), and it can store energy reserves in case the natural energy cycles get interrupted (fat reserves). It is not long now, if the right random mutation happens by, before it also starts hardening some cells to give it more leverage and mobility—it discovers a skeleton frame.

As soon as these parts are together, and the organism can move itself around to find energy, and it has been set into a cyclic pattern of spending and conserving energy; then the adaptations which will win out will be those which choose a scheme of "hunting" and "resting" that times itself well with the planet's natural circadian rhythms. It is now that the organism has become fixed into a "day" and "night" scheme.

Through all of this, there has been a common theme that thermodynamics will not allow it to forget. It must have one period that offers it a chance to take energy in, and one period that it is allowed to metabolize the energy it collected. That second period is what we on earth refer to as "sleep." For your planet, the life will not have these low energy periods come to it like our nights do; so only the ones who can move themselves to the night will be able to overpower the entropy of a nearly static energy level. This higher organisms can no longer get by on the weak energy from the planet's circadian rhythms that powered your early life. Perfectly stationary organisms like many terrestrial plants may not do well on your planet. Instead, the producers on your planet will need to happen across their own nyctinasty solutions which create their own high and low energy periods, likely by opening up to energy in their "day," and sealing up their mass in some bark or shell when they need to metabolize. Very early on, it would need to look much like modern flowering plants.

This leaves the more energetic life forms to use their mobility to migrate daily into the hunting grounds of the daylight-side of your twilight zone, and then return to the darker side, or perhaps a cave or the shade of a mountain, to sleep.

  • $\begingroup$ "because everything here has had the benefit of a very clearly defined diurnal cycle" - Those of us who live above the Arctic Circle wave hello and chuckle. $\endgroup$ Commented Mar 7, 2022 at 21:14
  • $\begingroup$ @KeithMorrison I don’t think evolution stuck you there? Point taken. $\endgroup$
    – Vogon Poet
    Commented Mar 7, 2022 at 21:19

Many people would have no great difficulty sleeping in a wide range of situations. I'm one of them. There is no reason to think that the normal sleep mode could not be like this.

I used to have trouble getting to sleep.
30 mins to one hour to more was not unknown.
THEN we had a 8 weeks premature daughter, my wife could not breast feed her and was permanently exhausted - after a while we found that she had easily curable pernicious anaemia.

I had to bottle feed my daughter twice a night. She would stop sucking with the teat in her mouth. If I removed the teat and she would cry continuously. Feed faster she would not. I was writing up a Master's thesis during the evenings and working full time.

Sleep I needed. After a while my body "learned" how to fall asleep in under 5 mintues, anywhere under almost any conditions. That was about 40 years ago. The fast drop off to sleep ability has never left me.
I'd rather bright light was not shining on my face. Loud noise is Ok - sudden bursts may annoy. I have slept under a grand piano in a Hong Kong hotel lobby. On consulting trips to China when a quick nap helps a lot, I have slept in hidden niches under conference tables, under a stairwell, and various other interesting places.

Your creatures should be able to do likewise :-)


How would life on the planet sleep?

Why would you necessarily assume that life needs to sleep at all?

"Sleep" is something that higher animal life on Earth do (and no one knows why); if life evolved independently on your planet, why would you necessarily assume it would get the same trait?

Now, for story reasons, you might want to assume that they do - that's your choice. However, assuming that your life has no need for sleep is an equally valid choice.


Simple: You have sleeping time be determined by when exhaustion sets in and whether the location is safe rather than light levels.


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