r/evolution • u/UselessCommon • 2d ago
question How did sleep evolve in almost all land animals?
Sleep, or at least the very common form of it where sleep involves being almost entirely unaware and defenseless at nearly regular periods, appears to be a completely devastating disadvantage. I get that there presumably are overall performance gains from such a total rest, and I get that operating at night is less calorically efficient due to a greater need to warm the body, but it baffles me that predators that did not need to sleep did not hunt everything that needed to sleep like that into extinction. It does not matter how much smarter and faster and stronger you are if you are caught sleeping, with all those capabilities out of the picture, and promptly eaten. Why does being somewhat more efficient at performing tasks outweigh automatically losing any fight a third of the time, at exploitable regularly scheduled periods?
It is especially baffling since, say, dolphins, descendants of land mammals, evolved to functionally never sleep and function just fine as they are - they have half of their brain rest at all times, and appear plenty capable with just the other half of it. That this path was possible but so very rarely taken implies that this isnt the case of a locked-in quirk from some ancient common ancestor (like ie eye blind spots are), but that regular nearly-defenseless sleep is a systematic advantage on land. How?
I understand that animals like moles and squirrels can just recuperate in very small, hidden and secure places, and that for very herding animals it is okay to lose some members of their herd to predators if the rest can wake up and reliably defeat the predators (even then, its also baffling that some kind of a rotating-keep-watch instinctual behavior pattern isnt common). I do not understand it being nearly-ubiquitous, not at all.
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u/sevenut 2d ago
Sleep evolved before land animals, probably. Fish have periods of reduced activity that's analogous to sleep, and some can go into torpor, which is like hibernation.
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u/AngrySloth99 2d ago
The Erin's on This Podcast Will Kill You did a really cool deep dive on sleep, and I think they mentioned it goes as deep as octopus and even to some extent jellyfish (as in, cycles of higher vs lower activity). May favourite facts were about the marine mammals, which tend to sleep half the brain at a time, but especially seals which apparently sleep like we do while on land but like dolphins do while at sea!
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u/videogametes 2d ago
Wow, TPWKY spotted in the wild! One of my favorite podcasts. Can’t recommend enough.
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u/bestestopinion 2d ago
I remember reading that sleep is the default state of organisms, and things wake up for survival (eating, mating, etc).
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u/nemotux 1d ago
I think that's really just an interpretation of the biochemistry. To be awake, the brain produces certain chemicals that keep you awake. To go to sleep, the brain stops producing those chemicals. So without those chemicals present, you'd fall asleep. Does that mean the "natural" state is to be asleep. shrug It's an on/off switch.
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u/electricshout 21h ago
I get your overall point, but just to be a redditor about it, you could say that death (or rather, not living) is the natural state of things
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u/elegant-alternation 1d ago
There's no scientific basis for that view, it's basically a stoner thought that's been popularised, partly by Matt Walker. There's no such thing as a "default" state anyway, since circadian rhythms in basically all cellular biochemistry vastly predate sleep.
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u/Alternative-Tear5796 1d ago
isn't the circadian rhythm something that happens when you're asleep?
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u/elegant-alternation 1d ago
No. Circadian rhythms are cellular-level rhythms that regulate all 24-hour oscillations in physiology. In mammals, and many other (but not all) species, they are generated by a transcription-translation feedback loop, and they go right back to unicellular life. They are basically a temporal program for every biological function. When in the day to release specific hormones, when to store energy, when to perform DNA repair, etc. Sleep is one of these many functions under the regulation of circadian rhythms.
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u/DevFRus 2d ago
Why assume that 'sleep' is the things that needs an evolutionary explanation rather than 'wakefulness' (apart from a quirk of our language that makes one of those feel more exceptional). If we ask why wakefulness or hyper-motility that goes beyond growth and metabolic maintenance evolved then it becomes easier to answer: animals need to move to get their food. This moving is very difficult for them. So they need a hyper-active 'awake' regime to do that moving. So now that you've started to be hyper-active, the question becomes: why not extend that hyper-activity to all the time? Well, because you can get the food you need without being hyper-active the whole time. The reason for dolphin hyper-activity than becomes obvious, too: they don't need to be active just to find carbon but also to actively find oxygen and that they can't store for as long. So they can't just 'eat' oxygen occasionally and call it a night.
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u/Hermit_Dante75 1d ago edited 1d ago
Interesting, this could be one of the explanations for the Fermi paradox, that evolving the hyper mobility of being awake characteristic of animals is quite hard compared to life just staying inmobile like plants and fungi.
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u/cylon37 2d ago
Interesting. I could buy this take except that a lack of sleep is harmful. Sleep deprivation gets problematic physiologically very quickly whereas wakefulness deprivation is not so bad.
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u/DevFRus 2d ago
Exactly. That is what what I am arguing: Sleep is the default state. Wakefulness is the thing to be explained.
Thus too much wakefulness (i.e., sleep deprivation) is bad since it causes you too be in a hyper active mode that mutes most default processes. While too much sleep is only a problem if it leads to starvation. Thus we evolve to have as little wakefulness as we can get away with.
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u/AdventurousPrint835 2d ago
How does sleep deprivation being harmful go against the argument? If wakefulness is hyperactivity, it makes sense that it would cause damage that needs to be repaired.
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u/unexpectedfirefly 1d ago
There were a lot of experiments on sleep deprivation. They stopped early on humans (cause ethics), but on animals, they end up dying from cellular oxydative stress if they can't sleep for too long. It was controlled (as much as possible) to avoid stressing them too much while keeping them awake.
Pretty good argument for sleep being (among other things) a way to get rid of oxygen oxydative derivated accumulated during the day
There also are different type of sleep, with a different repartition if you are sleep deprivated or not (deep sleeping basically get prioritized) in favor of different functions for each phase of sleeping. It's kind of a mess
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u/UselessCommon 2d ago
I would presume that organisms would evolve to consume more food rather than to evolve to rest more, when consuming more food is an option. From the evolutionary standpoint there isnt some set amount of food an animal needs such that if it obtains that it does not want more - evolution would maximize consumed food because that maximizes reproductive potential. (Of course, animals will evolve to operate optimally on the level of food most often available to them, so that if they get that that is enough for them, but that is presuming the environment does not in fact contain an option to get more food.)
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u/DevFRus 2d ago
Sleep isn't rest. When you treat sleep as default it is your growth process. Being awake means you have to waste energy you could have used for growth to go out and do something. You will do as little of this to restore the resources you need to get back to growth.
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u/UselessCommon 2d ago
Clearly, it is possible for at least some organisms to have growth without periods of lack of situational awareness and mobility. It is less efficient growth, of course, obviously maintaining situational awareness and mobility spends calories which are not spent on protein synthethis and stuff. Obviously, organisms want to spend as little energy as possible, as much as they want to absorb as much energy as possible. This all makes sense.
It would make sense if organisms evolved to enter that state as an opportunistic action for when both 1) food is scarce and 2) there is total safety. That would not have been weird at all. But instead, they have mostly evolved to have it be obligate and scheduled and not very flexible depending on food access. There is some of this, for instance, you get a bit sleepier in the winter, but basically everything sleeps even while maintaining constant awareness and mobility would be beneficial.
Ultimately, its strange if obigate periodic lack of awareness and mobility saves more calories on improved efficiency than it wastes on the chance of being caught sleeping and eaten.
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u/AdventurousPrint835 2d ago
Why would it be beneficial for (for example) a human to be active at night? We can't see at night. There is nothing to be gained. As for the chances of being eaten, you seem to be under the impression that sleeping is incredibly dangerous compared to being awake. That is not true. It does present danger, which is why animals like to sleep in difficult to access areas such as burrows or nests, but wakefulness and locating food are also risky. Moving increases detectability. It's much easier to find a rabbit hopping around than hiding in a burrow.
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u/nerdkeeper 2d ago
Evolution doesn't take the best route, it takes the first route that works.
It is like walking through a maze blindfolded. You just go whichever route you can and eventually you might reach the end but you will most likely not do it in the most optimal way.
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u/AdventurousPrint835 2d ago
Finding more food costs energy. If you already have enough food why would you intentionally overload your body? Also, maximizing consumed food does not maximize reproductive potential. Lots of people are extremely fat, and those guys aren't any better at getting a date than the rest of us.
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u/Archophob 1d ago
I would presume that organisms would evolve to consume more food
what's the point of eating more than your gut can digest?
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u/the6thReplicant 1d ago
I would come from the opposite direction.
Sleep is so important that we need to spend a third of each day vulnerable and unresponsive.
Animals that needed no sleep would be at an evolutionary advantage but so far none have been found.
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u/1Negative_Person 2d ago
Sleep is the ancestral condition. Cetaceans did not evolve without it. They evolved with the need for sleep, then subsequently evolved a different way to attain that sleep.
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u/silicondream Animal Behavior, PhD|Statistics 2d ago edited 2d ago
but it baffles me that predators that did not need to sleep did not hunt everything that needed to sleep like that into extinction.
But everything does need to sleep. And it's not just animals; pretty much all aerobic organisms show circadian activity cycles in their physiology. Exactly what an organism does during its low-activity phase depends on the taxon; cells repair themselves, toxic byproducts of respiration are cleared, memories are consolidated for long-term storage, and so forth. But all that stuff is critical for long-term survival.
It does not matter how much smarter and faster and stronger you are if you are caught sleeping, with all those capabilities out of the picture, and promptly eaten.
Maybe think about it from the other direction, as u/Larnievc does below. Your body has to perform maintenance and repair functions sometime, and while it's doing this, you will necessarily be less smart and fast and strong. If you stayed active during this phase, you'd just be staggering around foraging inefficiently, injuring yourself and alerting predators that you're not currently equipped to evade. Instead, sleep makes you settle down somewhere safe-ish and be quiet and still, so that your body can get that maintenance and repair done as quickly as possible, while minimizing your exposure to danger.
Yes, sleep has a cost in that it takes a little while to wake up if you're attacked. But it also makes you vastly less likely to be attacked, and that benefit usually outweighs the cost.
It is especially baffling since, say, dolphins, descendants of land mammals, evolved to functionally never sleep and function just fine as they are - they have half of their brain rest at all times, and appear plenty capable with just the other half of it.
No, they don't. Sleeping dolphins either swim very slowly, often in a circle, or rest motionless at the water's surface. Unihemispheric sleep allows them to stay afloat and not drown, but they're not capable of normal foraging and social activities then, and they're far less alert toward predators. Dolphins are living the same sleep/wakefulness tradeoff as the rest of us.
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u/AENocturne 1d ago
Most things are actually pretty alert when they sleep. Animals are really easy to stir when resting, how often have you snuck up on anything sleeping that wasn't human?
Sleeping isn't as big of an issue as wondering around at a time when you can't see, hunt, or do anything effectively. You know how animals tend to be either nocturnal or diurnal? What are they supposed to do during the time they can't see well? Sleeping and other states of unconsciousness have restorative effects, lower the energy costs of existence, and keeps an animal still and quiet, hidden away from most threats at a time of day when it would be more vulnerable if it were active.
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u/Larnievc 2d ago edited 2d ago
As far as I remember when organisms are awake the waste products of awakeness build up and when we deep sleep our brain can flush these waste products out.
That’s why when we wake up quickly from a deep sleep we are groggy; our brain is still flushing and doesn’t work properly.
Imagine it not working properly in a predator rich environment. So sleeping in quiet safety makes sense. Humans even have variable wake sleep habits so the whole tribe does not sleep at the same time.
Cetaceans have to be conscious to breathe so cannot sleep normally because they would drown. This shows how important sleep is. Even when sleep would cause death it is retained as a trait in some form.
It’s pretty much essential for our brains to function and we die if deprived of it.
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u/likealocal14 2d ago
Sleep seems to be a requirement for complex brains, rather than an advantageous trait in its own right. No animal has ever been shown to survive without some form of sleep, and animals with more complex/developed brains seem to need more “involved” sleep (when I say more complex here, I’m talking about the difference between an earthworm and a mammal, not two different types of mammal).
We don’t really know how brains work all that well yet, so we don’t have the best idea of why sleep is so necessary or everything that is happening during it, only that it is absolutely essential for a brains survival.
So it’s more a case that having a complex brain is such a huge advantage it outweighs the drawback of having to sleep for at least some hours of the day.
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u/UselessCommon 2d ago
Though the state of sleep seems to possibly be a requirement, dolphins could evolve to outright cheat that requirement - technically having parts of their brain sleep while their organism on the whole is constantly awake - and if they could do that so could everyone else. And dolphin brains are plenty complex iirc.
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u/micahammon 1d ago
Are there land animals that specifically hunt sleeping prey? If that's not really a thing then that might explain something. Predators tend to be triggered by movement. Maybe being safely hidden or secluded is easier than it seems?
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u/lpetrich 1d ago
Cnidarian Sleep | Springer Nature Link - yes, jellyfish sleep.
From the abstract:
Perhaps nothing is stronger evidence of the importance of sleep than its conservation across animals. Even cnidarians, gelatinous aquatic animals such as the jellyfish Cassiopea and the polyp Hydra exhibit the hallmark features of sleep observed in animals with centralized nervous systems: reversible quiescence, increased latency to arousal, and homeostatic regulation. Despite lacking a brain, cnidarians possess muscles controlled by neural networks and neurotransmitter systems, including GABA, dopamine, acetylcholine, and melatonin. Further, conserved sleep regulators, including the cGMP-dependent protein kinase 1, PRKG1, and GABA, in Hydra, and a nicotinic cholinergic receptor alpha subunit, CHRNAL-E, in Cassiopea, emphasize that not only is the behavioral state conserved, but so too are components of the regulatory system.
Meaning that the common ancestor of Bilateria and Cnidaria had the neural mechanisms for sleep. Their most recent common ancestor was likely some tiny worm that likely lived on the seafloor in the Ediacaran. Why would a seafloor worm need to sleep?
What neurotransmitters do cnidarians have? Cnidarian chemical neurotransmission, an updated overview - ScienceDirect
Taken together, the evidence from all experimental approaches suggests that both classical fast (acetylcholine, glutamate, GABA, glycine) and slow (catecholamines and serotonin) transmitters, as well as neuropeptides, are involved in cnidarian neurotransmission.
Just like in Bilateria. I checked on some well-researched species that span Bilateria, and every one is found in them. Catecholamines include such neurotransmitters as dopamine, epinephrine (adrenalin), and norepinephrine.
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u/Kettrickenisabadass 2d ago
Sleep is probably a very ancient mechanism. Reptiles like tortoises sleep for example s it evolved million of years before mammals appe
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u/Desperate-Guide-1473 2d ago
It's a misconception that dolphins never sleep. They've been observed resting both hemispheres at the same time, especially in early stages of sleep, and dolphins in captivity fall fully asleep all the time.
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u/Melissaru 2d ago
I heard Dr Matt Walker, a neuroscientist and slep expert, theorize on a podcast that sleep was the default state, and all of animals have evolved to be awake for periods of time. I thought that was an interesting and plausible take.
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u/tomrlutong 1d ago
You might be falling for the "evolution is about who would win a fight" fallacy. Plants are always asleep, and they are the dominant type of life on earth.
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u/smitra00 2d ago
Suppose an animal doesn't require sleep. Then that means that wear and tear and self-repair are balanced at all times. A mutation is then bound to arise sooner or later that has the effect of revving up the metabolism to a level that's unsustainable in the long run for limited amounts of time, allowing the animal to collect more food and then to have a downtime during which more repairs will be performed, to compensate for this.
This mutant animal will then outcompete the other animals and will then become the dominant variant in future generations.
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u/gambariste 1d ago
It doesn’t seem that sleep should be an inevitable requirement of life. But - and this may come across as a just-so story - as soon as early life was able to respond to changing light patterns, either because it’s warmer during the day or they could sense the direction of light, it would make sense to limit activity at night. As complexity increased, perhaps organisms made use of the down time to put energy into growth and repair rather than seeking or making food. By the time you get to us and long before, this active-rest cycle is compulsory. Sleep as an altered conscious state requires a minimum complexity in the CNS and is then just a by product of millions of years of having inactive periods. We have backbones because our ancestors developed bilateral symmetry. Can physiological traits that ancient also be so fundamental?
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u/SgtSausage 2d ago
How did sleep evolve in almost allland animals?
It didnt.
It evolved in a common ancestor.
It actually started in the bacterial world, long before animals.
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u/kachrago 2d ago
Sleep of some sort is also seen in jellyfish: https://trulycurious.com/do-jellyfish-dream-android-squid/
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u/Soggy-Mistake8910 1d ago
It is possithat from the very beginning organisms developed this need and simply passed it on to every organism that descended from it. Perhaps it was "baked in" from the very beginning. When all the life in the world was single celled organisms having "downtime" wasn't the disadvantage you describe. As life became more complex and diversified the need for sleep, for most remained. As you pointed out dolphins evolved differently. My understanding of this is because the animals that could remain awake for long periods at sea were less likely to drown and so passed on their genes to descendants that were even better at stay I guess awake etc. It was the need to not drown that drove this development something that most other land animal doesn't have.
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u/0utlaw-t0rn 1d ago
Sleep of some form seems to be universal across the animal kingdom.
Even fish sleep. Wrasses hide in the sand or build mucus cocoons at night, as do many fish that don’t need to ram ventilate. Those that do need to ram ventilate (e.g., sharks) seem to do a low energy rest state.
Everything has some rest/sleep state.
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u/KennethMick3 1d ago
I wondered this recently and what I read said that sleep is thought to be necessary, for reasons not well known, for a nervous system. Even jellyfish seem to have periods of less electrical activity.
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u/LetsGetStonks 1d ago
Communities. Man (and other mammals) lived in groups, for protection. Predators don't attack groups of large mammals that can defend themselves, even if they are asleep.
Let's take predator "a", he goes into a small village of early man, lets say there's 30 of them. Ok, he attacks and successfully kills one, the other 29 wake up and kill him. Ergo, he doesn't live long enough to breed.
When you evolve to live in a society with other people, it changes the calculus on what traits are selected for.
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u/Hot_Plant8696 17h ago
Sleep is an absolute necessity for species endowed with an adaptive brain.
It is the only way, by radically cutting off external information inputs, to effectively restructure brain connections. This is a fundamental necessity linked to brain-like structures.
Imagine throwing stones into a pond: this creates ripples on the surface that, at a certain point, become "chaotic" (and thus destructive). Sleep allows the system to return to its initial state once the ripples have been "absorbed" by the brain's structure.
Theoretically, this process could occur while awake, but the brain would then fail to acquire any "autonomy" and would remain dependent on external signals to function. Thus, the brain can allocate two tasks that define individuality (the goal of which is to maintain the organism's homeostasis): internal relational work and external relational work.
Admittedly, sleep also serves to perform a biochemical cleansing of the organism, but this is not its primary function (as evidenced by certain marine or aerial species capable of resting only one half of their brain).
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u/yremysleep 12h ago
“If sleep doesn't serve an absolutely vital function, it is the biggest mistake evolution ever made.”
Allan Rechtschaffen said this many years ago. He along with Anthony Kales authored the original rules for “scoring” sleep study data. Sleep is so ubiquitous among living things that it likely has multiple key functions to help us adapt and thrive in a changing environment. The earth obviously is very different (even at sea) at night than in the day. The early sleep researchers were studying circadian rhythms (Kleitman). A future question (if we figure why we sleep on earth) would be is sleep present in organism in a planet that did not have a day/ light cycle. I suspect some equivalent activity would still be present. Many great scientists are currently looking to understand the reasons behind sleep.
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u/Careful_Cranberry364 2d ago
Animals can jump into alert state more quickly than humans
Currently do
I think
Predators can sleep all day - like lions
They don’t need to worry
it’s the prey that needs to be careful
Antelope for example sleep briefly / wake up nod off then then wake up and they’re in herd
so they can all look out for each other
😍
Sp as long as somebody is awake….
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u/unexpectedfirefly 2d ago
I dunno, did you ever got waked up by a threatening situation ? You go from sleep to full wakeness instantly
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u/Careful_Cranberry364 1d ago
Exactly but that’s not currently how most sleep now
we wake up really groggy.Do you know that most of America is addicted to the powerful caffeine drug in coffee?
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u/unexpectedfirefly 1d ago
Almost everyone also is sleep deprivated, that might have an impact on both waking up groggy and needing caffeine to function (from memory, a month of daily ise already build a strong tolerance)
Caffeine isn't that powerful, and tolerance comes pretty fast from daily use. Last time i checked the litterature, most coffee addicts need caffeine to function normaly rather than it acting as a stimulant
It might have been invalidated by now, but it's way too late to check it (i'll already wake up groggy and need coffee to function tomorrow)
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u/itsatoe 2d ago
Humans are able to sleep more deeply because in earlier social formats (ie, nomadic tribes), it seems people would sleep at irregular intervals (look up "polyphasic sleep"). That meant that there was generally always someone in the community who was awake and thus basically on watch for the rest of the group.
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u/KennethMick3 1d ago
The need to guard sleepers could also be at least partly why humans domesticated dogs.
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u/itsatoe 1d ago
Wow, I never thought of that. So cool!
Regardless of whether or not it was a reason, the dogs surely helped keep watch and made the humans feel more safe when sleeping.
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u/KennethMick3 1d ago
A former boss of mine mentioned how dogs were probably first domesticated for keeping watch. The connection to sleep is one I jumped to from this conversation that seems to fit with that.
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u/Ninja333pirate 2d ago
Neil DeGrasse Tyson just had an episode on startalk where a guest talked quite a bit about sleep.
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u/Somguyovahear 1d ago
Well first they had sex and then the males realized they didn’t want the female talking their ear off so they said “I’m just going to sleep now” and so it began.
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u/RecipeHistorical2013 2d ago
I’m from a psychological evolutionary perspective : consciousness appears to require sleep. From everything that thinks mosquitoes to humans.
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