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.
Siegel, J. (2009). Sleep viewed as a state of adaptive inactivity Nature Reviews Neuroscience, 10 (10), 747-753 DOI: 10.1038/nrn2697
30 comments:
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.
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!
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http://noamgr.wordpress.com
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.
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http://noamgr.wordpress.com
You could have made a joke at the end about sleeping with the fishes. Maybe next time.
But dolphins aren't fishes :P Although I suppose they do sleep near fishes (if they sleep).
I forsee bad times ahead for the unfortunate dolphins when this research swings into action.
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.
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.
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.
http://www.sciencedaily.com/releases/2009/09/090915174506.htm
maybe this is why?
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.
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.
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.
> 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.
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.
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?
Eric
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.
Thank goodness we sleep, imagine how much damage we would have done by now if we didn't sleep!
Anon, no purpose is served by the behavrioral definition of sleep. What you should do is take the EEG definition and run with it.
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).
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.
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.
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Cheers,
-A
Sleep - "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?"
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.
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.
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.
A valuable post on fitness.
Thanks,
Karim - Mind Power
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.
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
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.
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.
Parker - That's a really interesting point.
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