Why Do We Sleep?

By Neuroskeptic | September 19, 2009 7:37 pm

Why do we sleep? Because otherwise, we’d always be doing stuff.

This is the theory advanced by UCLA sleep researcher Jerome Siegel (website) in a new paper, Sleep viewed as a state of adaptive inactivity (free pdf). It’s part of a Nature Reviews Neuroscience special issue on the evolution of the nervous system. Siegel proposes that the evolutionary function of sleep is simply to ensure that animals are only active when the benefits of movement (mostly access to food, and mates) outweigh the costs (activity burns calories, and puts you at risk of predation or accidents).

Sleep, in other words, is our equivalent of the inactive states into which most living things, even plants, periodically enter when it suits them. Even (deciduous) trees spend the cold, dark half of the year doing not very much. In Siegel’s view, this is their equivalent of sleep.

This theory stands in contrast to the idea that sleep has a restorative function – that animals need to sleep, because some kind of important biological process can only occur while we’re sleeping. This idea is intuitively appealing – it feels like we benefit from sleep, and at least in humans sleep deprivation has many well-documented negative effects.

But, as Siegel points out, we’re far from any kind of a consensus on what the biological function of sleep is. It’s generally assumed that there is one, and a great many have been proposed – he lists some, ranging from that sleep is important for the formation of new neural connections, to the idea that sleep is needed to reverse cellular damage caused by oxidative stress (interestingly, Siegel himself contributed to one of the papers he gives as a reference for that idea).

If a vital restorative function of sleep were to be conclusively identified, Siegel’s theory would obviously be disproven. On the other hand, if Siegel is right, several things should be true. Firstly, the proportion of time that an animal spends asleep should be directly proportional to the amount of time that it is useful for it to be active.

Siegel argues that this is what we find. The big brown bat for example is the doziest of all mammals, sleeping for 20 hours per day. But it wouldn’t benefit from being awake any more, because the insects it feeds on are only active for a few hours at dusk. If it were flying around during the day, it would just be wasting energy (and risking becoming lunch for a bird.)

By contrast, he says, some marine mammals (cetaceans, dolphins and whales) never sleep at all. In land mammals, sleep consists of distinct periods of neural activity such as REM and slow wave sleep. Neither, however, occurs in cetaceans. They do show a kind of neural activity called Unihemispheric Slow Waves (USWs). But these are confined to one half of the brain at a time. It’s often said that this is “half the brain going to sleep”. However, the animals remain moving normally, and are able to avoid obstacles, during USWs. It’s not as if only half their body remains awake. As such, Siegel says, the USW state is not sleep.

If it’s true that there are animals which never sleep, this is strong evidence for Siegel’s theory, and against the idea that sleep plays a vital role. But not everyone agrees with his claim that dolphins and whales don’t sleep. See, for example, this 2008 open-access paper, Is Sleep Essential?, which calls Siegel’s theory of sleep the “null hypothesis” and then proceeds to criticize it.

In particular, the authors claim that dolphins do sleep, albeit with only one half of their brain at a time, and they make the interesting point that “the very fact that dolphins have developed the remarkable specialization that is unihemispheric sleep, rather than merely getting rid of sleep altogether, should count as evidence that sleep must serve some essential function and cannot be eliminated.”

At this point the debate becomes highly technical. The sleep behaviour and neural activity of marine mammals is hardly easy to research, and it looks as though more evidence is needed before we can know for sure whether they sleep or not. This is one of those seemingly trivial questions which could end up deciding between two theories with enormous implications. There are quite a lot of them in science. We don’t yet know why we sleep. But the answer may lie with the dolphins.

Link: More recently, I asked Why do we dream?

ResearchBlogging.orgSiegel, J. (2009). Sleep viewed as a state of adaptive inactivity Nature Reviews Neuroscience, 10 (10), 747-753 DOI: 10.1038/nrn2697

  • http://themadandwild.wordpress.com/ themadandwild

    I dunno. The fact that every creature has some kind of downtime, even when they need to be alert all the time, that is, they don't function at the same capacity all the time, seems to support the idea that there must be some restorative function to it.

    Unless, of course, you can argue the fact that the brain is just stripping itself down into it's most basic mode required. However, then why do dolphins alternate which side of the brain is sleeping?

    I'm not sure, however, why the two theories contradict each other. 'Idle time' could easily explain the length of sleep, while restorative function could explain why we have this.

    I guess it becomes a chicken and egg problem. Did evolution cause animals to target certain groups, thus have 'most active times', or did the 'most active times' cause evolution to favour sleep at certain times? The answer, I'm pretty sure, would be both.

  • http://noamgr.wordpress.com/ noamgr

    I'm not sure I buy it either. I can see some form of “proto-sleep” evolving early on to serve as nothing more than downtime while the organism is better off not doing anything; but it's clear that, if this is the case, it has since become absolutely essential for the brain. It just makes no sense that even the slightest sleep deprivation should be so dangerous if it serves no other purpose.

    I mean, skipping just two or three sleep sessions is incredibly dangerous; it can even be fatal.

    An animal can go for days without food and still come out perfectly fine– which means skipping a dozen or more “eating sessions,” if we assume this animal is meant to eat a few times a day. In this sense, whatever sleep is, it seems it is tantamount in its importance to breathing: it's part of a cycle that is so essential to the organism that it must not be disrupted it at any cost.

    Hell, your brain will try and complete the cycle even if you're in the middle of life-threatening activities such as driving if it must!


  • http://noamgr.wordpress.com/ noamgr

    Oops, I cut of this part for some reason:

    Since sleep does seem to differ so much between species, as he points out in his paper, it does make sense that initially sleep would serve no other purpose.

    But it makes just as much sense that this “idle time” soon proved to be the perfect moment to take on various tasks (restorative? memory consolidation? etc.), and that these tasks are now as much an important part of the organisms' life cycle as is breathing.


  • http://www.blogger.com/profile/12758457878736361701 Rob

    You could have made a joke at the end about sleeping with the fishes. Maybe next time.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    But dolphins aren't fishes :P Although I suppose they do sleep near fishes (if they sleep).

  • http://www.blogger.com/profile/17687910584661433398 Cavall de Quer

    I forsee bad times ahead for the unfortunate dolphins when this research swings into action.

  • http://www.blogger.com/profile/14647896216499813443 Kapitano

    There's another hypothesis – that most animals spend their whole lives “sleepwalking”, but a few have developed periods of wakefulness as a precondition for intelligence – retaining sleep either as a spandrel or co-opting it for some other use.

    For that matter, there's the possibility that sleep once served to enable some function to occur, but the function atrophied or disappered, leaving the now pointless 6-8 hours unconsiousness every day.

  • Anonymous

    The piece of trivia that does my brain in is this.

    Fur seals sleep like terrestrial mammals when they're on land but have USW when at sea, like dolphins.

    The argument about whether dolphins and whales 'sleep' or not is a bit tautological. On one hand we have a EEG-based definition which indicates that USW is sleep on one side of the brain. On the other hand if they respond to stimuli in a purposeful manner with both sides of their bodies then they fail the behavioural definition of sleep.

  • http://www.blogger.com/profile/05956474344809716833 Kamil Szot

    Dolphins do sleep. But only with half of the brain. Other half remains active. Then the other one sleeps and previously sleeping is awaken.

    One of the functions of sleep might be just inactivity but another much more important one must be associated with neurons.

    I think sleep (namely REM phase) is needed for neural network to avoid overlearning. That's why sleep deprived persons hallucinate. Their neural network is overlearned to recognize stimuli from senses and it recognizes known patterns even if stimuli is week. Nothing better to avoid that is to disconnect neural network from muscles (to avoid hurting yourself) and discharge neurons randomly for some time.

  • Anonymous
  • Anonymous

    You're mixing up two questions: “why did our ancestors evolve sleep?”, and “what function does sleep serve for modern humans?”. There is no reason they should be expected to have the same answer; once the sleep cycle gets baked into the genome, then other adaptations can start to take advantage of it, and so it will develop additional functions that have absolutely nothing to do with its original evolutionary purpose.

    If we had evolved from non-sleeping ancestors, then we would presumably have evolved some form of intelligence that didn't require shutting down every 24 hours. But we didn't.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Anon 17:50 – That's a good point, although Siegel (and other researchers) seem to conflate those two issues as well.

    On the other hand, if all animals sleep, sleep presumably serves a similar function in humans as it does in other animals. There's nothing special about human sleep biologically speaking, we have REM and non-REM sleep just like most other mammals. So it's unlikely that sleep plays a unique human role.

  • http://www.blogger.com/profile/01436580484351596781 David

    Well, there are plenty of anecdotal accounts of people who have adopted the Uberman sleep schedule (or the less rigorous 'Everyman') who have totally concluded that REM is the only required stage of sleep–and that after a brief 20 minute recharge of pure REM cycle sleep, they feel as good as new–often better. They sometimes report minor but strange changes in dietary cravings, which is often assumed to be the body's way of restoring whatever functions or nutrients might naturally have happened or been generated in sleep phases 1-4, which no longer occur. So, at least for the most intrepid of us, much of sleep has been shown to be, in fact, useless.

    My personal experience slipping into a lucid dreamstate from consciousness involves a total disconnection from the senses and control of my body. I think that whatever our consciousness might be, even as a part of our brain, it needs a break from the arduous task of directing billions of cells and nerve impulses to stay active and mobile. It's less our body's rest, in my opinion, and more our mind. It shakes itself free of the encrusted finitude of physical movement and temporal constancy, so that it can continue to direct the body in space and life upon waking.

  • Eric Johnson


    I would call it sleep if their performance was better with the whole brain turned on. Or if there was no performance difference, but it could be shown that their encephalization was higher than one would expect from their intelligence: this might suggest they had increased their brain size, in part, precisely in order to prevent a performance deficit during unihemispheric sleep.

    As for the overall question, Noamgr's comment convinces me that the “null hypothesis” is wrong.

  • http://www.sleep-sound.com Sleep

    If we don't need to sleep then it is strange we take it because we are at risk when we sleep. We have no way to protect ourselves from predators when asleep.

    The theory put forward that sleep is not essential is hard to swallow. Why do we feel so ill when we miss a lot of sleep and so much better when we have slept if sleep is not essential to us?

  • Anonymous


    You can't just change the behavioural definition of sleep to suit your hypothesis rejecting purposes. Behavioural sleep is a dissociation from the external world and it's stimuli. If the dolphin is still swimming around avoiding other dolphins and other features of it's environment then it isn't asleep according to the behavioral definition. The problem is that it meets the definition of sleep as measured by EEG.

    Neuroskeptic. I don't think that Jerry Siegel is conflating the reasons for the evolution of sleep in the first place and the function of sleep now. It may not be clear in that particular article but I'm pretty sure he's the one who taught me that particular distinction in the first place in something of his I read.

    On the subject of the phylogeny of REM sleep though. It may have evolved twice as REM is also found in birds but not in reptiles.

  • Anonymous

    Thank goodness we sleep, imagine how much damage we would have done by now if we didn't sleep!

  • http://www.blogger.com/profile/17584502524397663834 Eric

    Anon, no purpose is served by the behavrioral definition of sleep. What you should do is take the EEG definition and run with it.

  • Eric Johnson

    In order to benefit mankind, I'll elaborate a little. In the evolution business we must reduce everything to fitness, which comes of course from function. If I hypothesize that sleep exists to rest neural tracts, then resting neural tracts is the function; being unconscious is not a function, it's a side effect of resting neural tracts. Only if I think being unconscious has a function do I need to be concerned with the behavioral definition of sleep and be surprised by the unihemispheric sleep of dolphins. Since I myself don't think that being unconscious at night has a function, I need only be concerned with EEG-defined sleep, and with explaining why it is economical for dolphins to do it one way, land mammals to do it another (see above).

  • Anonymous

    Seems like an important question is whether all creatures that sleep have brains, and all creatures that don't, don't. (Seems to me that ceteans do.) Is that the case? If it is, then you could at least infer that it's likely that sleep does something beneficial for the brain, and you could relate levels or types of brain activity to the amount of sleep across creatures that sleep.

  • http://www.blogger.com/profile/18078462764857337905 Alejandro Montenegro-Montero

    Very interesting post and some very provocative discussions in the comments section!

    I've selected your blog post as one of my “picks of the week” of molbio blog posts aggregated at ResearchBlogging.

    Check it out here: http://bit.ly/vOF1V


  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic


    I think this is a crucial question. as I said, the sleep-is-restorative theory is intuitively appealing because it feels right.

    But Siegel could account for this.

    If sleep exists to make us stop moving, then it will only work if it really makes us want to stop moving. There would be no point in sleep, if we could just run around for six days without sleep just because we felt like it. If sleep is there to make us inactive, it has to make us inactive by making it progressively more unpleasant to stay awake.

    A good analogy is with physical pain. You might say “Why does pain have to hurt? Couldn't we just become aware of pain, and decide to act to avoid damage?” But this wouldn't work. Pain has to hurt because that's what makes us avoid damage.

    Technically we could survive without pain, and in some situations it would even be an advantage (pain is a distraction), but overall it is better to feel pain than not.

  • http://www.blogger.com/profile/08461338194309128443 Paul

    I don't believe this theory sufficiently explains REM sleep and dreaming. If we are required to sleep only for the reasons suggested, why has evolution got to such lengths to safeguard REM/dreams. Indeed, REM sleep deprivation can have many physiological/psychological consequences.

  • Eric Johnson

    Neuroskeptic I read Siegel's article, which I hadn't until now. Certainly, some of the considerations are perplexing, such as the relative suspension of sleep in manic humans.

    Your question about why pain has to hurt is interesting, but I'm not sure it's “admissible to court” in my mind – does it not verge awfully close to the mire, the boundary zone of science? Without free will and the intuitive free ego, it seems that pain wouldn't be fitness-enhancing in the way you outline. So it seems the line of reasoning rests on a doctrine I would call a more-or-less philosophical one (though I certainly don't reject it with confidence).

    Also, your logos on behalf of Siegel doesn't seem to explain the severe degradation of basic awareness and performance that sleep deprivation causes. It seems that after millions of years of the existence of sleep, we would have reached a fitness-optimized state where sleep deprivation was highly unpleasant, yet _not_ so stuporous that your terrible Foe can easily blindside you with a spear in the guts.

  • http://www.blogger.com/profile/10113177931970999545 karim

    A valuable post on fitness.

    Karim – Mind Power

  • http://www.meaningofdreams.org Marcia Dream

    I'm not sure if I can accept the behavioural definition of sleep. I've responded to stimuli – in what to others, would appear to be a purposeful way – while asleep, for example, I've had conversations with other people when I was asleep, with the other people being deceived into thinking that I was awake because my answers made logical sense in the context of the conversation. People sleepwalk, sleepeat, sleepdrive, etc.

    To me, what defines sleep is the state of consciousness. Whether or not dolphins are sleeping does not depend on whether they are reacting to stimuli in an expected manner. If people can sleepwalk, then dolphins should be able to sleepswim.

  • http://www.blogger.com/profile/09946556071500977272 Interior design ideas

    l suffer from sciatica, somtimes it is really chronic, l was advised to buy a memory foam mattresses so l could get a good nights sleep, and memory mattresses do make a difference

  • Parker

    Maybe the dolphins are what we would think of as 'sleep walking' in the sense that they recognize their movement but aren't completely aware of it.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Eric – I agree that the pain argument is a bit philosophical, and I'm not sure I buy it 100% myself, but it is quite persuasive. For better or worse we feel things like pain and hunger and thirst, we don't just think “I need calories”. That's a hard fact. Maybe fatigue is the same.

    The fact that severe sleep-deprivation makes us so incompetent might seem evolutionarily counter-productive, but I wonder whether extreme sleep deprivation (like 48 hours or more) would ever occur in nature. Recently, people have done it, out of curiosity or for scientific purposes. But then, people also use contraception, which from an evolutionary point of view is absolutely absurd – evolution didn't expect us to invent contraceptives.

    Again, that's quite a philosophical argument but I think this is the nature of the question.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Parker – That's a really interesting point.

  • Anonymous

    If sleep is merely dormancy than why is it unconscious? What animal benefits from being vulnerable and unconscious as opposed to just sitting still for a few hours until it can hunt again? The fact that most animals lay there helpless for a few hours every day rather than remaining alert enough to look out for themselves suggests that sleep must serve some purpose critical enough to offset the risk of 5-8 hours a day of vulnerable unconsciouness.

  • EG

    Reading all this I'm still torn.
    You would expect the selective avantage of wakefulness – opportunities for eating/ finding mates / being aware of predators? – would be very high, putting those organisms who slept the least at a distinct survival and reproductive advantage over their more dormant relatives. Even if sleep is a default state I wonder why this pressure hasn't removed it or reduced it down to a tiny amount of the day.

    Is this the case?

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    EG: I think the claim is that being awake is only good at certain times – if you're a predator, and your prey spend all night hiding in holes (because it's dark so they can't see you coming), you gain little by being out. You're better off asleep. As are they. Although in that case you might find the predators sleeping during the day, and creeping into holes at night… but you get the idea. Whereas being awake is costly, in calories, at any time.

  • http://www.blogger.com/profile/15470961423480709366 Jordan C

    I think sleep definitely is a regenerative function. If it isn't, how come we feel so depleted of energy when we go to bed and so charged after we wake up? Even the tiniest amount of sleep can give us energy.

  • Jean- Baptiste

    Hi, here me out, this is awesome :)

    I've been studying memory systems for years now and I know for certain that the number One biggest purposes of sleep is processing memories by clearing out the Emotion that's stored within those memories.

    I think its very weird that every article or professional you read or talk to about sleep, they have little to nothing useful to say about emotions and sleep.

    It really is a big deal. If you can recall any memory and still feel an emotional charge in it, you haven't cleared it, your not getting enough delta sleep and memories of traumatic experiences are the worse.

    You know why that is such an even bigger deal? It's because that energy is what Your body would and should be using to operate.

    Even though your not always consciously aware of all the memories with the emotional charges (positive or but mostly negative) attached to them, you're subconcious is very, very aware of these charges.. always.

    It's energy that your body would normally use to give you confidence, more Go, life, health, vitality but instead like pretty much everyone on the planet right now, you're assigning it to memories.

    It's the number one reason why we get sick, we get tired, we get old and we die.

    Have you noticed, you could be in a good mood but as soon as something or someone upsets you, pisses you off – boom! you're mind's no longer in the moment, you feel off guard , a little slower thinking, not as sharp. Your energy drops.

    About the age of 3 years old the body makes an internal calculation that its storing more energy during the day than it's freeing up at night and that's when it realises that it's dying. you can tell with kids when they starting saying “that's mine, this is mine' instead of sharing their toys without identifying with them.

    And obviously it gets worse as you go through life the more negative shit you keep holding on to.

    We need Delta sleep, it's the deepest however Alpha is where the emotional clearing happens, your brain activity is generally 8-13.9 Hz in Alpha. Electricity interferes with that like crazy.

    There's heaps of stuff you can do to manually clear out the memories, loads of techniques on the internet. Start with the big memories and I garuntee you will notice a massive, instant transformation. You'll get a rush of energy and that's yours now. The next day, wow, Literally your whole life will change for the better the moment you clear those big memories.

    I hope someone has found this useful because it's true and I hope I haven't pissed any one off because I'll just be giving you extra work for tonight :)

    Thank you for reading this.
    Have fun!

  • Anonymous

    Rial, R.V., et al., Evolution of wakefulness, sleep and hibernation: From reptiles to mammals. Neurosci.
    Biobehav. Rev. (2010), doi:10.1016/j.neubiorev.2010.01.008

  • Anonymous

    Rial et al. The trivial function of sleep Sleep Medicine Reviews (2007) 11, 311–325

  • http://www.blogger.com/profile/15579272589604671824 lucy sunshine


    Then what would explain waking up more tired and unrefreshed, as many people do?

    Neuroskeptic – what a fun blog you have here! Thanks for sharing your insights.

  • Anonymous

    I've thought about this. If we get our energy from food, why can't we eat our way to feeling rested?

    I think that food provides us with the parts our bodies need to continually rebuild themselves and some maintenance energy.

    My theory is that the real energy source that drives our consciousness has yet to be discovered and we disconnect from our physical bodies to recharge our consciousness in this energy field.

    Our bodies can only handle so much of this energy, so we have to recharge a bit everyday.

    Just a way out there way of looking at it. I'm probably way off the mark, but something fishy is up. People die without sleep there is a genetic disease called fatal familial syndrome (something like that) You die because your brain can't switch you off into sleep.

  • Anonymous

    If he was right then wouldn't we not have dreams when we sleep? If its just to save energy and nothing else then wouldn't dreams waste that energy.

    But we know that dreams can sometimes help people. Sometimes you might be stuck on something, sleep on it (literally) and wake up and have it figured out. Sometimes you might even remember the dream that gave you the idea.

    We even know what causes dreams partially. DMT, which is produced naturally in the brain and also exists as an illegal drug and a religious sacrament for some groups. People see intense hallucinations and sometimes epiphanies or they can realize things from reflecting on the trips.

    So it seems part of the function of sleep is to immobolize us so our imaginations can completely run wild to the point of hallucination without putting us in danger. Considering that if you don't sleep long enough you start hallucinating anyways this function clearly serves a critical purpose.

  • scottk

    I think that sleep serves a definite biological function in the brain.

    Sleep is a critical part of your mental and physical well being. Sleep has a direct affect on your mental and physical function. Sleep provides time for the brain and body to regenerate, re-sensitize and re-balance for the days activities. The main function of sleep is to restore the brain and body back to the form that it started the morning. Sleep is divided into two main stages, Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM). REM sleep is mostly associated with dreaming. It is called paradoxical sleep because much of your brain is activated including the Cognitive, Emotional and even the Autonomic centers while the consciously controlled muscles are inactivated. These same centers are turned off or resting during NREM sleep. The sleep states are essentially restoring neurotransmitter supplies and re-sensitizing receptor during resting and rebalancing their activity during sleep. The more excitatory centers are activated during REM sleep with their inhibitory centers resting and re-sensitizing. The expression of the excitatory centers during REM sleep desensitizes the responsive systems. This decreases responsiveness to excitation which is like cutting out the background noise in the system. The activation allows for rebalancing of repressed centers that might otherwise express during waking. During the NREM sleep stages the inhibitory centers are more activated and the excitatory centers are resting and regenerating. This essentially does the same for the inhibitory systems desensitizing their targets to inhibition. All of these stages fall within a set pattern expressed through the night alternating from one stage to the next

  • Anonymous

    My question is if sleep is universal. Every living thing on earth sleeps in a 24 hour cycle because of the rotation of the earth; daylight and nighttime. Would animals on other planets sleep? Would they sleep on a cycle comparative to their planets rotation around their sun? If the human species continues to evolve for millions of more years, would no sleep evolve as a better thing for the species? Do aliens sleep?

  • CreatorOfInfinity

    well neuro path ways are the primary system for which we store and process all information, mainly memory since all thought is based on memory in some form, and seeing as memory can be increased in functionality by simply doing what is known as “Chunking” or “Mind mapping” in which data can be crunched into a smaller concept, and usually visual image, with simple logic, one can crunch chunks, and increase the speed of memory recall, and reduce the energy and “storage space” needed to complete these processes, there fore extremely decreasing the work load that is needed, on could then simply calm their mind and slow down when appropriate to finish off the little work left over, second; any damage done to cells require nothing but energy light and oxygen which are readily available, and with your processing speed increased, you can easily realize that food is readily available for humans as we have dominated the earth lmfao, and light ( actually its light energy, glucose, and oxygen which means glucose supplements would do.) and well you get plenty during the day, so that's easy plus sunlight energy can be simulated with technology, in its necessary essence it is ultra violet radiation. Also since i agree that sleep is a simple survival tool, in which we have transcended the need for due to our neo cortex, which spawned reason and understanding. the biggest problem is when youve slept so often you send conscious decisions to your sub conscious in which to fix a chemical, time to sleep nudge. Over ride that urge and teach your subconscious to change its patterns.

  • http://geniusphotos.com Tommy

    The reason we sleep is very simple: The body and mind need to recuperate after a hard days work and needs to repair itself. If we did not sleep we would get bored resting and not rest properly! Its that simple!


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