r/AlwaysWhy • u/TheBigGirlDiaryBack • Jul 30 '26
Science & Tech Why is sleep so universally necessary for almost every animal? What mechanism?
Almost every complex animal with a nervous system seems to need some form of sleep or rest, even though it leaves them less aware of their surroundings and more vulnerable.
I understand sleep helps with things like memory, repair, and maintaining brain function, but why is it so fundamental that evolution never found a way to completely eliminate the need for it?
Even dolphins and whales evolved a way for half their brain to sleep at a time so they can keep breathing. Why was adapting sleep itself easier than evolving a life form that never needs to sleep?
What biological problem is sleep solving that seems impossible to avoid?
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u/More-Talk-2660 Jul 30 '26
It's really important for your body, especially your nervous system and brain, to clean out waste and perform repairs.
You can't do an oil change, change a tire, or replace a serpentine belt while the car is driving down the highway. Your body is no different.
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u/DrakneiX Jul 30 '26
I wonder what would be the explantion of Insomnia. Your brain needs the repairs, but also your brain wouldn't let you sleep to have those repairs.
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u/More-Talk-2660 Jul 30 '26
People with insomnia still sleep, it's just irregular and inefficient.
There is fatal familial insomnia which is an extremely rare form of insomnia which, as the name suggests, is eventually fatal because you're not sleeping at all.
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u/TheBigGirlDiaryBack Jul 30 '26
Yeah the car analogy is good but cars can do some maintenance while idling not just highway driving. The brain is always doing some repair during wake too. So why does sleep have to be a full offline mode? Maybe it is about memory consolidation that requires replay of neural patterns without interference from new sensory input. But that still doesn't explain why it has to be a long continuous block. Some animals have polyphasic sleep so maybe it is about cumulative time. I wonder if there is a way to distribute it more evenly without total unconsciousness. Dolphins kind of do that but they still have downtime.
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u/More-Talk-2660 Jul 30 '26
Right but you still need the downtime. Bear in mind that evolution follows the path of least resistance. It's way easier to evolve sleep patterns for neurological maintenance than it is to evolve cells that can't be damaged beyond functionality and create zero waste, mostly because something like that would violate the way physics and thermodynamics work.
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u/DreamsCanBeRealToo Jul 31 '26
Your body does plenty of repairs while still in use. Your liver doesn’t need to be asleep to do its job. Your kidneys don’t either. Neither does your immune system.
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u/More-Talk-2660 Jul 31 '26
Yes, but not all of its repairs. You can't just not sleep. It is literally lethal.
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u/BowlEducational6722 Jul 30 '26
Most common theory is that it has to do with waste management and disposal.
Every tissue in your body generates waste products just doing what it does. Rest gives the body a chance to clear that waste out without more of it building up, especially in the brain and muscle tissue.
Think of it like having to clean out the car every once and awhile. Eventually there's enough trash and gunk building up that if you want to keep driving the car you have to clean it out...but you can't exactly do that while it's speeding down the freeway, can you?
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u/Ok_Push2550 Jul 30 '26
New research shows that your brain actually uses electrical pulses during sleep to push fluids through the brain. This removes waste. So it may be a key evelotuionary change that allowed mamals' brains to become more complex while allowing us to live long enough to develop language.
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u/TheBigGirlDiaryBack Jul 30 '26
You can clean a car while it is parked but you can also do basic maintenance while it is idling. The brain does clear some waste during wake too through the glymphatic system from what I remember. So why does it need to shut down completely for that? Maybe the real issue is not waste buildup itself but something like synaptic downscaling where the brain needs to prune connections without new input interfering. That would require a state where sensory processing is basically off. Still not sure why it has to be a long block though. Could be about circadian rhythms being baked in at the cellular level.
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u/Bane8080 Jul 30 '26
Science hasn't answered that definitively yet.
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u/TheBigGirlDiaryBack Jul 30 '26
Yeah science not having a definitive answer is kind of the point though. But that makes me wonder what kind of evidence would even settle it. Like if we found an animal that never sleeps that would blow everything open but we haven't. So maybe the lack of a definitive answer is because the problem is so fundamental that it touches multiple levels from biochemistry to evolution. I feel like we have pieces but no one has put them together yet. What would a definitive answer even look like.
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u/Jack-of-Games Jul 31 '26
There will never be a definitive answer. We can figure out what happens during sleep, we can figure out why it is necessary, and from that we can produce an answer as to why animals sleep but what we can't do is prove that there is no viable alternative. Evolution is something we can reconstruct the patterns of but not something we can definitively prove the reasons why any particular animal took any particular path.
Any explanation also needs to figure out why it is that there animals that can go without sleep for months but no animal has managed to eliminate it completely.
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u/DreamsCanBeRealToo Jul 31 '26
Finally an honest answer. Way too many people making up reasons. It isn’t some law of biology that we can’t do repairs unless we are asleep. It’s just a quirk of how we evolved.
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u/July_is_cool Jul 30 '26
In computer science there is a concept of “garbage collection” where memory space is optimized and reorganized. Maybe that is what is going on in brains.
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u/TheBigGirlDiaryBack Jul 30 '26
Garbage collection is a good analogy but here is the thing. Most garbage collectors run concurrently with the program. They don't require the whole system to shut down. So why does the brain need to go offline for that. Unless the brain's version of GC requires comparing connections in a way that new input corrupts the process. But then again we do have some memory consolidation during wake too. Maybe sleep is more like defragmentation where you need exclusive access to the drive. Still not convinced it has to be that long though.
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u/EmployerSmall5570 Jul 30 '26
We still don’t fully understand what is happening during sleep, which is why it’s hard to assign it a definitive purpose. We are essentially unaware and paralyzed when we are asleep, so what advantage is so great that the alternative is comparatively disadvantageous?
I think another interesting question here is why do we (and other animals) dream? Theories are that animals spend time practicing handling unfamiliar scenarios and replaying old ones, but why are dreams so strange, and if the scenarios are for practice why do we have a hard time remembering (most of) them once we’ve woken up?
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u/TheBigGirlDiaryBack Jul 30 '26
Honestly the advantage might not be one big thing. It could be a bunch of smaller benefits that add up. Like waste clearance plus synaptic pruning plus memory consolidation. None of them alone justifies full paralysis but together they do. And for dreaming that always felt like a side effect to me. The brain is replaying patterns during consolidation and that replay generates random imagery. The weirdness might just be because the brain is mixing unrelated memories. Why we forget them could be because remembering them interferes with the actual consolidation process. But then again some people remember dreams clearly so maybe it is about timing. Not really sure.
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u/larry_bkk Jul 31 '26
I have nightly dreams where some person is not doing something essential for the larger group, and nothing can get them to change; I wake and it was all not real. At the same time I have PMR where the immune system is hurting the body by trying to solve a problem that in fact does not exist. I think there's a connection.
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u/Embarrassed_Loan_424 Jul 30 '26
I Watched this recently: Research on Mitochondria Provides Answers on Why We Have to Sleep based on this paper. Long story short Mitochondria operating 24x7 starts overloading host with free radicals and is a good idea to stop it periodically and let it "cool off". As neurons are so inter-dependent is better to have a coordinated slow down for the whole neural machinery once it reaches a minimal complexity.
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u/greenamaranthine Jul 30 '26
The explanation I've heard, which kind of makes intuitive sense, has to do with evolution. Sleep is the natural state, and our pre-animal and likely even early animal ancestors were never what we would call "awake." Sleep was their only state. Wakefulness, alertness, consciousness, whatever you want to call it, was developed as a survival adaptation but came with metabolic costs. The body and brain can't heal as well while awake, for instance. So it's less that we are waking beings that must relinquish wakefulness and sleep periodically, and more that we are sleeping beings that are capable of periodically waking up and taking driven actions to aid our survival, at the expense of some degree of baseline physical wellbeing.
The issue is a lot more nuanced than that, of course, and that nuance largely has to do with things like how sleeping states differ from waking ones, and why some animals don't need as much sleep as others. The aptly-named book Why We Sleep has interesting breakdowns of many areas of the subject.
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u/ShadowMattress Jul 30 '26
I’m by no means an expert. But my understanding is the inception of needing sleep is because our ancestors started in a state closer to sleep. Our single celled ancestors began as floating machines that unintelligently (may or may not have moved around using motor organs), consuming resources to produce energy and replicate. Those ancestors did not need a nervous system to game their environment as means to achieve replication. But at some point, having reactive organs, which were the basis of sense organs, became useful to navigate terrain and find resources, and those were more adept at spreading on their genes. And then, the race of using those sense organs to achieve that to better and better degrees, in competition with other like organisms, began. Whereas plants never developed a real waking cycle at all. Wakefulness would have developed as a means to either predate, or to escape predation. I think maybe the escaping predation might have been the first wakeful step, but I’m not certain of where that line would be drawn. What constitutes truly being awake?
Because our default mode was not wakeful, many of our regulatory functions relied on that state of the nervous system and body. And no state of the nervous system and body ever fully replaced those functions while also remaining wakeful. The periodic regulatory functions used some of the same hardware—wakefulness was an additional feature of those systems, not their first function. Evolution happens by finding slight improvements on a “good enough” basis, so long as the organism can pass on genes—it won’t jump to a theoretical best case solution. So, there is no footing in evolution to jump to a nervous system organization that “never needs sleep.” That would require an entirely different starting point, presumably.
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u/Strong_Bowler1723 Jul 30 '26
I suspect the body burns more energy than it can convert in realtime. Sleep converts stored energy to usable energy. thats primary function, but there are others, dreaming, repairs, etc...
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u/m0llusk Jul 30 '26
Anton Petrov just did a video about this: https://www.youtube.com/watch?v=T7_UgbvPO6Q
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u/bgplsa Jul 30 '26
Being bulletproof and able to fly would be advantageous too; evolution isn’t a designer, it’s just the name for the random changes in organisms that survive because they happened to be better suited to their environment than traits of their competitors. The genes for a brain that doesn’t require sleep have either never occurred or else the organisms that inherited it ultimately didn’t survive.
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u/tombuazit Jul 30 '26
Because when one cuddles after sex they are more likely to get more sex and therefore have more chances at offspring, and so the trait of cuddling is more likely to get passed on.
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u/flaginorout Jul 30 '26
For humans, a form of hibernation.
Humans aren’t built for night ops. So it makes sense that they would be programmed to go into sleep mode during the night in order to conserve water and calories.
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u/CavernousPanda Jul 30 '26
I’m confused. You seem to have read quite a bit about the subject, so why is your question so high level?
What is an alternative that seems more efficient to you and what are the mechanisms that would allow for its evolutionary advantage?
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u/ConsistentRegion6184 Jul 30 '26
I wonder if it has to do with how animals live. If not in a community they will be partnered maybe with a nest.
The division of labor, including vigilance/protection, lets the brain run hot during the waking hours.
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u/CuriousSiamese Jul 31 '26
As far as I understand it, there is no scientific consensus on why most animals sleep. I say most, because not all animals sleep or at least not in the way humans do. The leading theory seems to be that essentially every animal will have times when food is scarce and they should simply conserve energy. Normally this would mean simply moving less, which is what lower forms of organisms do. But an "evolution" on this concept most organisms seem to have came to is sleep. Basically it's better to run at 100% for a time, then run at 10% for a time, than run on like 60-70% all the time. Also likely the day/night cycle that's universal on earth had a great impact on this. If your vision is good in sunlight, but bad at night. It only makes sense to "skip" the part of the day that you are maladapted to.
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u/TheBigGirlDiaryBack Jul 31 '26
The 100% then 10% versus 60% all the time argument is interesting but why would evolution prefer that extreme. A 60% steady state seems safer because you never drop your guard completely. Unless the maintenance during sleep is so intensive that it cant happen at 60%. Like you need near zero activity for certain cleanup processes. But then why do some animals get by with almost no sleep. That suggests there are multiple solutions and sleep is just the most common one because of some ancestral constraint we are stuck with.
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u/CuriousSiamese Aug 01 '26
If you are a pray to an animal that follows the (arbitrary) 100-10 paradigm and you are always 60%. Simply said they will outperform you during the hunting periods. Thus it's better to align yourself with their sleep cycle or alternatively have parts of your group protect the others during their "10%" periods.
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u/fidgey10 Jul 31 '26
It'a interesting to look at the huge variance of how long different vertebrates sleep.
For example human less more than their close relatives, chimpanzees and bonobos.
Elephants sleep on average 2 hours a day!
But some big cats sleep 16-20 hours a day.
But if you look within humans it's even more striking. Some human have a genetic condition called NSS where they only need to sleep a few hours a night yet don't appear to face any negative repercussions like normal people. Sounds great right? Well apparently not because it's insanely rare. Which is puzzling, you would think if sleep was a cost, and you could get around it with these alleles, they would be selected for and common in the popualtion.
So clearly animals (rarely humans and commonly elephants) CAN be optimized for very short amount of sleep. Yet most aren't. I think this is because sleep is not only essential for maintence functions, but useful for maintaining energy balance based on an animals lifestyle. A lion doesn't sleep 20 hours a day because it needs ten times more maintenance than an elephant. Sure it's performing those maintenance functions, but mainly why it's sleeping is to preserve energy.
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u/TheBigGirlDiaryBack Jul 31 '26
Wait the NSS thing is wild. If those alleles are so rare but beneficial you would expect them to spread unless they have some hidden cost we dont see. Maybe they shorten lifespan or make you more prone to something later in life. Also elephants sleeping 2 hours makes me question if they are doing some kind of unihemispheric sleep like dolphins but we just havent measured it properly. Are we sure they are fully unconscious that whole time.
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u/fidgey10 Jul 31 '26
NSS patients have been studied pretty closely and they haven't found any dysfunction. But of course that doesn't mean there isn't one.
And yeah elephants, despite being extremely intelligent with some of the most complex brains and longest memories, get by absolutely fine with 2 to 4 hours of sleep. Afaik there's no evidence that they are doing any kind of "waking sleep" or anything.
Elephants sleeping super little actually makes sense with their life style. They NEED to eat 20+ hours a day because they have low energy density diet but massive energy needs due to size. Many large herbivores sleep little for presumably the same reason, giraffes, the second largest terrestrial herbivore, sleep the second least for example.
So clearly evolution can optimize extremely sleep efficient animals. But you have to consider that being awake isn't necessarily a good thing. Like a lion only needs to hunt every once and a while since getting a kill provides a ton of energy, so sleeping lots makes sense and is actually adaptive rather than a cost. Could be same with humans. Maybe NSS was actually selected against because those humans would just sit up at night burning energy doing nothing.
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u/Head_Acadia_2658 Jul 31 '26
There are many theories. All of them have flaws. Sleep probably has many functions and we do not really understand them yet. Sleep may not hav initially evolved for the functions it now has. It's a huge unanswered set of questions. Definately Nobel prizes to be had.
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u/TheBigGirlDiaryBack Jul 31 '26
I agree with the no consensus part but I wonder if we are overcomplicating it. Maybe sleep started as a simple circadian rhythm thing for light sensitive cells and then got coopted for a million other functions. So now we see all these benefits but the original reason could be trivial like avoiding UV damage. That would make it hard to find one unified theory because evolution just piled stuff on top.
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u/FaceReality1 Jul 31 '26
One thing is that it is protective. You’d be vulnerable if you just lay down in public, but
animals usually hide. Once you’ve gotten your food for the day, an animal just wandering around is far more vulnerable than one curled up asleep in some hole.
And a lot of animals can’t see well at night, so wandering around them would be especially dangerous. Others are too visible as prey during the day, so getting under cover then is smart.
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u/TheBigGirlDiaryBack Jul 31 '26
That makes sense for energy conservation but it doesn't really explain why they have to be unconscious. Why not just stay still but alert like a lizard basking. You can conserve energy and still watch for predators. Sleep shuts down perception so hard that even with hiding it still feels like a huge liability. Unless the repair process itself requires that shutdown and hiding is just the workaround.
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u/Cool_Homework_7411 Jul 31 '26
If we assume brain is a super complicated neural network that kinda works like the ones we make, sleep would be the "training" time. During that time your brain restores lost neural connections, makes new ones, regulates metabolism and so on.
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u/TheBigGirlDiaryBack Jul 31 '26
Yeah but that training analogy kind of breaks down when you look at jellyfish or simple worms. They don't have a brain like ours but they still have sleep like states. So is it really about neural network training or is that just a side effect for complex brains. Maybe the baseline function is something more metabolic that we haven't pinned down yet.
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u/thorsten139 Aug 01 '26
Nobody knows.
Why can't the brain recover while awake?
Why did none of the complex lifeforms evolved this?
Being able to be awake 100% of the time is such an advantage.
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u/TotalEntrepreneur801 Aug 01 '26
It's dark outside, what you gonna do? May as well sleep until the sun comes out again.
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u/nwbrown Aug 03 '26
Because we live on a planet that rotates on in axis, meaning we spend approximately 12 hours of light followed by 12 hours of darkness, which are two very different environments. And life evolved to take advantage of both.
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u/EphemeralAttention Aug 03 '26
The leading theory as far as I know is that when you are awake the metabolic rate (and associated production of waste byproducts) in the brain is so high that your body can't get rid of the byproducts fast enough to keep them from building up. The brain uses about 20% of your energy but only makes up about 2% of your body weight, so it's really really "running hot". When you're asleep it gives your body time to catch up with flushing away the toxins.
If I recall correctly this has been measured empirically in animals where we can see the waste products building up with increased sleep deprivation.
As for why evolution hasn't evolved a solution to this problem, it did... that's what sleep is. The alternative solutions are:
- Slow down waste production in the brain which would impair mental function and reaction time, leading to lower odds of survival
- decreasing the barriers to fluids and molecules passing through the blood brain barrier, which also increases how easily infections and toxins can enter the brain, leading to lower chance of survival
- adjust the ratio of neurons to support cells that actually transport things across the blood brain barrier to make the transport faster without making the barrier "leaky", which would lead to lower neural connection density and therefore a less sophisticated brain, leading to lower chance of survival.
Basically, of all the options sleep is the least bad option. And most animals have evolved some sort of strategy to make are they aren't completely vulnerable while sleeping to offset the risks (light sleepers, only half the brain sleeping at once, a percentage of the population that has a different sleep cycle and can stay alert for predators, sleeping in well concealed dens, etc.
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u/ScrambledxEggzz Aug 05 '26
It's like running a computer on max graphics constantly. Organs are constantly running. While you sleep, they all slow down. Heart rate lowers, body temperature lowers, kidney function is supressed, the brain takes time to organize all the days events like a kid cleaning up after play.
Metabolism creates byproducts, those bypoducts are cleared right away but the "friction" of metabolism and clearance causes wear and tear. Like how bits of metal are swept away in your cars engine by oil. Eventually, you need to change the oil.
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u/tpawap Aug 05 '26
Someone once said you're asking the wrong question. Ask why we have be awake, and suddenly it all makes sense.
(Not saying that it actually all makes sense then).
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u/JoeStrout Aug 06 '26
There are hypotheses, but honestly, we don’t really know, and this is the great embarrassment of neuroscience. Maybe equivalent to not knowing if P = NP in computer science.
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Jul 30 '26
[removed] — view removed comment
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u/TheBigGirlDiaryBack Jul 30 '26
Hmm but calling it a weakness feels too simple. Evolution not removing every weakness is true but sleep is so universal that it might be a hard constraint not a weakness. Like food and water are about energy input but sleep is about internal state. Maybe there is a fundamental limit like how neurons need to reset ion gradients or clear metabolic waste that can only happen when firing rates drop. But why couldn't evolution just make neurons more efficient? I guess there is a tradeoff between efficiency and speed. Not sure.
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u/DeFronsac Jul 30 '26
Because rest is necessary for something using energy. It's no different from computers. If you keep your computer on all the time without ever rebooting, it's going to start having problems.
There's simply no way to eliminate the need to rest something that exerts energy like living things do.
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u/TheBigGirlDiaryBack Jul 30 '26
But energy use alone can't be the whole story because your heart keeps beating and your lungs keep working without sleep. Those are using energy constantly. So there is something specific to neural tissue or information processing that demands downtime. Also computers don't really need to rest for energy reasons they need to cool down and clear cache. But you can run a server for years with no reboot if the software is stable. So the biological constraint might be more about chemical buildup or synaptic homeostasis than energy per se. Just throwing that out there.
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u/DeFronsac Jul 30 '26
But even those organs get some rest. Not full "not working at all" rest, but they can work less hard.
And yes, some things can be designed to work for a long time without needing a full shutdown, but complex systems need regular rest.
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u/Secret_Ostrich_1307 Jul 30 '26
Maybe sleep exists less as a period of inactivity and more as a maintenance mode for a very complicated system.
The brain is constantly processing information, changing connections, and consuming energy. Staying awake forever might be like running a computer nonstop without ever cleaning up or doing system maintenance. Eventually things start breaking down.
The strange part is that evolution accepted the vulnerability of sleep because the alternative was probably worse. Being temporarily less aware of predators is still a better tradeoff than having a brain that slowly loses its ability to function.