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What is the real reason we sleep? (bbc.com)
156 points by sfled on Mar 21, 2016 | hide | past | web | favorite | 84 comments

Very relevant PubMed article many of you will undoubtedly find interesting: "Partial sleep in the context of augmentation of brain function."

Abstract: Inability to solve complex problems or errors in decision making is often attributed to poor brain processing, and raises the issue of brain augmentation. Investigation of neuronal activity in the cerebral cortex in the sleep-wake cycle offers insights into the mechanisms underlying the reduction in mental abilities for complex problem solving. Some cortical areas may transit into a sleep state while an organism is still awake. Such local sleep would reduce behavioral ability in the tasks for which the sleeping areas are crucial. The studies of this phenomenon have indicated that local sleep develops in high order cortical areas. This is why complex problem solving is mostly affected by local sleep, and prevention of local sleep might be a potential way of augmentation of brain function. For this approach to brain augmentation not to entail negative consequences for the organism, it is necessary to understand the functional role of sleep. Our studies have given an unexpected answer to this question. It was shown that cortical areas that process signals from extero- and proprioreceptors during wakefulness, switch to the processing of interoceptive information during sleep. It became clear that during sleep all "computational power" of the brain is directed to the restoration of the vital functions of internal organs. These results explain the logic behind the initiation of total and local sleep. Indeed, a mismatch between the current parameters of any visceral system and the genetically determined normal range would provide the feeling of tiredness, or sleep pressure. If an environmental situation allows falling asleep, the organism would transit to a normal total sleep in all cortical areas. However, if it is impossible to go to sleep immediately, partial sleep may develop in some cortical areas in the still behaviorally awake organism. This local sleep may reduce both the "intellectual power" and the restorative function of sleep for visceral organs.


What if the question was phrased as what is the real reason we wake up and dedicate some conscious time for our mind? Surely, because in the physical world we live we need to arrange for some time to deal with the unavoidable practical things such as finding food and shelter. Then, after a long enough period of this activity focused on these very tangible, physical and physiological constraints we can finally say we fall to sleep again.

You are saying we wake up just to survive ("finding food and shelter"), but it's right in the article:

“The cost of losing consciousness to survival is astronomical,”

"This means we can confidently reject one of the simplest theories of sleep: that we drift off simply because we have nothing better to do."

Doesn't make sense to me. To take care of those needs, we don't need to be awake nearly as much as we are. On the other hand, staying awake all the time would have moderate efficiency benefits and provide a lot more protection from threats. So we should either be sleeping a lot more or barely at all; the middle ground has no benefit under this model. The model of sleep as recovery works a lot better for the durations we see.

Isn't that basically what the scientist asks in the last paragraph?

> “What about this hypothesis: sleep was the first state of life and it was from sleep that wakefulness emerged,” says Walker. “I think it’s probably a ridiculous hypothesis – but it’s also not entirely unreasonable.”

Perhaps to that question the answer is: to secure nutrients and safety for our body as we sleep?

We can't all be plants and fungus.

Plants seem to sleep as well, can't pinpoint the exact source right now, but it was from a documentary. Something among the lines of "the hidden lives of plants"


I think it is possible for life to evolve on a planet with only plants and fungus.

Plants have been pretty successful down here on Earth, and I think most of them don't need animals for their survival (except flowers?).

I think it is possible for life to evolve on a planet with only plants and fungus.

That's kinda circular, given that plants and fungi are examples of evolved life :)

You are correct that plants don't strictly need help with pollination (they simply release their spores to the wind/current and hope they land on a receptive partner), however the odds of a succesful pollination decrease sharply as the number of competing plants in an area increase.

In my (uneducated) view, in a scenario where all life is stationary (rooted to the ground, hur hur), you are much more likely to find monocultures -- where most plants can interbreed -- or more species using asexual reproduction.

Flowers don't necessarily need animal life either, though it does help. Wind can pollinate.

More importantly, plants evolved in the context of animals; if animals didn't exist, plants wouldn't have evolved seed dispersal or pollination strategies relying on them. They might have even evolved strategies we've never seen, that don't work well when there are animals running around eating everything.

It's too late for that now, for now.

I took Matt Walker's class on sleep at UC Berkeley (Psych 133) and it was fascinating. Short-term effects of a lack of sleep are easily observable, and I'm sure we can all relate from personal experience. However, long-term effects of sleep deprivation are harder to determine. One of the few known correlations appears to be that people who are susceptible to psychiatric disorders such as schizophrenia are more likely to experience them as they age, if they suffer from chronic sleep deprivation.

"Sleep problems may increase risk for developing particular mental illnesses, as well as result from such disorders." - http://www.health.harvard.edu/newsletter_article/Sleep-and-m... (2009)

This is an interesting one. I have sleep apnea, the result of which is basically chronic sleep deprivation even though you spend lots of time in bed. I struggled with all sorts of depressive symptoms (depression, anxiety, panic attacks) for years before getting a diagnosis, and 90% of the symptoms vanished overnight after treatment.

I tend to believe it's tied quite closely to memory and emotional states. I had very bad sleep apnea that was untreated, and for the time that it was untreated my memories are extremely vague compared to most.

I also had very high anxiety which is completely gone now. Now that I use a machine to sleep my memory is sharp again.

I also eat much less now, so I believe that calories were a way the body would attempt to counter act the lack of rest.

These are just my thoughts, I honestly don't have any scientific background on it. Just my own experiences.

How did you discover you had sleep apnea?

Probably through an overnight clinical sleep study.

Another option is to perform a sleep study at home. You borrow a wearable device that records your sleep quality, breath, ECG, etc. You wear it over night, then turn it in for analysis.

Common symptoms of sleep apnea are if you're regularly tired without apparent reason or experience a reduced ability to concentrate during daytime, or if your surrounding complains that you snore loudly.

Careful though, as HST can under-report events, resulting in a lower AHI score than if you got the clinical PSG.

Interesting that you had the similar experience to mine – I had years of depression, panic attacks, and anxiety that vanished virtually overnight with sleep apnea treatment.

I don't use a machine though, just a brace.

Note that sleep apnea can also lead to oxygen depletion, so it could be difficult to discern which of those symptoms were caused by brain damage from lack of oxygen vs. sleep deprivation.

It's mostly sleep deprivation though. I graduated with a 3.7 from a top 5 school in CS after I was treated. My apnea was SEVERE before treatment: 30 apneas per hour.

I doubt I had any brain damage, though my study only recorded a min SpO2 of 90%.

Before CPAP, How many times do you actually remember waking up in middle of night?

Once or twice. Usually you don't consciously wake up in a way that allows you to remember it whenever you have an apnea, you just go into a lighter and less restorative sleep stage.

From someone else with apnea: absolutely none at all.

I didn't have to use CPAP to treat it though, so other people's experiences might be different.

How did you treat it?

I use a mouth guard, it turned out to be sufficient.

What machine do you use?

All of these answers for why we sleep - garbage collection in brain processes, energy recovery - beg the question, why do these processes HAVE to happen during sleep, instead of during wakefulness.

I suspect that it's an artifact of the Earth having a day-night cycle at all. Imagine the very first organisms on earth. One is always wakeful, and spends all of its awake time hunting for food. During the earth's night cycle, hunting for food is harder - it's colder, so smell molecules travel more slowly. It's darker, so you can't see food as well. The organism has to work harder to gain the same calories compared to during the day. If there's another organism that goes into a low-energy state during the night, it exchanges the risk of being killed for food during the night with the lower energy budget of not needing to be active at night. Even if a particular organism gets eaten, it's a net positive for the species, and these organisms can outcompete the wakeful organisms.

Then, as sleepfulness becomes part of the successful energy budget of early organisms, you can further optimise nighttime activities - organisms that do brain garbage collecting during the night can do so more efficiently than organisms that also sleep but do continuous garbage collecting. Because sleep was the optimal strategy for primitive organisms, fitting recovery into sleep becomes selectively chosen for.

The only way to prove this, though, would to find a planet that has no day/night cycle, and see if those organisms have any sleep behavior.

>The only way to prove this, though, would to find a planet that has no day/night cycle, and see if those organisms have any sleep behavior.

Deep sea hydrothermal vents provide this experiment on Earth. While the ecosystems are not completely isolated from our sun based ones, they are pretty insolated from it. Of course, to actually prove this you would need multiple, independent, evolutions of life with a day/night cycle and multiple evolutions without one.

It might happen.

I remember watching this TED talk a couple years ago which stuck with me, though am not 100% sure on its validity.


The theory being that sleep was a way to clear out waste from the brain during normal operation, due to the lymphatic system not extending to the brain, it has to put itself into "sleep mode" to drain the waste.

This might explain why memory is usually better after a good night's sleep, and also the cause of dreams - which is just misfiring of neurons while the waste is being cleared.

So sleep is something like garbage collection in Java? Some perceptions/thoughts you experience during the day need to be mived from a finite sized young generational heap to a long kived heap (ie convert important short term memories to long term memories). This then frees uo space in the young gen. heap for the next day. Dreams could potentially be the result of a reference check being performed to see which memories to move to a different heap and which can be discarded.

It's already in the article:

"Neuroscientists discovered a previously unrecognised network of vessels in the brain that flush out the fluids between brain cells: the “glymphatic system”."

Conjecture (half serious): we won't get strong AI until we figure out why we need to sleep.

Begins to sound like a science fiction novel where a bunch of sleep researchers discover that far from being 'down time' sleep is actually a period where our neural hardware is being used by beings from another dimension to perform some nefarious calculation. Or if Douglas Adams were still alive a P2P version of Deep Thought.

Relevant short story by Issac Asimov: https://en.wikipedia.org/wiki/Hostess_(short_story)

My (semi-serious) theory: Sleep is us logging off of this MMO game we play.

Our "real" bodies (minds) need some downtime from the game.

So this explains why real AI will never happen: This reality is not where our consciousness lies.

Wouldn't they just pause the game though? In which case we'd never notice.

Each person logs off at a different time, it's one game for everyone not a single player game.

Probably related. Mammals dream, and we humans experience time in dreams much faster than the actual passage of time. Dreaming is likely the same mechanism as 'experience replay' used in AI reinforcement learning. We're just training our neutral networks using minibatches of our experiences from our waking hours.

"Likely" is not the word I would go for. "according to my wild guess" is a more accurate term.

I've heard it before and it's not their wild guess but a seriously discussed idea. It's just that it's hard to set up experiments to test these sort of ideas.

It can be a seriously discussed wild guess. It's not like there's some dichotomy to be satisfied there.

I read somewhere a conjecture that sleep for human brains is analogous to regularization schemes (such as dropout) for neural networks. Dropout helps the net avoid overfitting; nets that are overfitted have a difficult time distinguishing noise from signal. Sleep for mammals might serve a similar purpose, to help the brain avoid overfitting to noise (imagine if you had difficulty distinguishing dreams from reality).

Yea, I would suspect this is true as well. Upenn had some sleep studies that were looking this idea, I'n not sure how mature the research was/is though.

Couldn't it go the other way, too? With weak AIs, we get a sandbox of rough sketches of consciousnesses with which to experiment. Some optimisation here or there which looks uncannily like sleep may shine light on the biological version.

I think you guys are interepreting carsongross a little too literally. The bigger point is we understand very little about sleep - we understand even less about how neurons learn and how the brain functions.

Might just be recalculating weights of the Neural Net for learning which can't be done with the system "on line" or concious.

Would make sense with the learning connection and memory.

My hypothesis is that all RNNs (and in general complex dynamical systems) need to be reset periodically. If run for too long without resetting, they tend to get stuck in strange states, blow up, or cease activating. You can see this effect by running a generative RNN model for a long time - eventually the output is garbage.

Under this model the next obvious question is why it takes so long to reset the brain's state. Maybe it can be done faster.

Maybe, maybe not. If it turns out it's only a biological cause (waste disposal, tired neurons, ..??), this would not be the case. But good point.

But not before we determine whether salt is bad for us.

I just find it the most amazing thing. 8 hours a day, we all do it, and we don't know why. It's akin to having part of the globe unexplored.

Anyone got any recommendations on lay-person books on sleep/subconscious?

Two things missing from the article: physical repair is improved in sleep; and the laydown of long-term memories is an important part of sleep. You can screw up someone's memory of the previous day by waking them at the right times.

And, as usual, REM sleep steals the show. Probably because of that funky acronym, and that's when we tend to have movie-like dreams. Wake people up at the right time in slow-wave sleep and they'll report 'dreams' that are just sounds or flashes of light.

Bonus factoid: you can wake most people up from even deep sleep by calling their name a few times. When I was a 'sleep technician', that's how I used to wake people up when I had to fix a sensor - much more gentle than physically touching or shaking them.

It's a bit disappointing that journalists are still peddling the "sleep is a great big mystery" angle. And also that the article is arguing that there must be one particular reason why we sleep. There isn't one reason why we respirate, one reason why we digest, or one reason why we circulate - why should there be one reason why we sleep?

In my mind, all of your other examples have a singular answer, even if the answer isn't the full story.

one reason why we respirate

To replenish the oxygen in our blood.

one reason why we digest

To extract energy from our food.

one reason why we circulate

To distribute nutrients around the body.

Simplified as those answers may be, they're not wrong FAFAIK, they're incomplete. Do we have such a single not-completely-wrong answer for why we sleep?

Well, the one thing you've missed in all the items is waste removal, which is just as important as nutrient distribution. Also, we absorb more from our food than just energy. And even then, you caveat your answers with "but these are simplified and incomplete".

Then compare to the article, a bit of long-form journalism rather than throw-away summary lines, which opens with the statement that there's one crucial reason for sleep and we don't know it.

One thought I've had recently: What if being "unconscious" isn't unconscious at all - what if it's a different form of consciousness?

For instance, it deals with the internal world of our body, maintaining and dealing with issues as they happen. It's responsible for releasing chemicals to get a desirable action from our 'conscious' self. It gives us rashes in an attempt to change its environment.

I haven't thought about this too much, but in the very least, I think it's an interesting thought experiment. Has anyone actually tested that our 'unconscious' is indeed unconscious?

Bad legacy firmware, same answer as always.

I like the idea of sleep as a consequence of information processing. At some point an organism needs to get rid of useless information acquired by its sensors previously. That's also why Maxwell's demon can't brake the second law of thermodynamics. Looking at people's behavior suffering sleep deprivation 'information overload' comes easily to mind. Perhaps the ability of REM sleep is a sign of some form of consciousness, which in turn is just an efficient way to process information and detect the useful bits.

This rambling summary article touches briefly on "natural selection" but no one has yet to look into what seems obvious. In mammals where most parents have to care for their young, if the young did not sleep, did not give their parents a break for long periods, they would not survive. Therefore sleep is selected for. Those offspring that sleep survive, passing on the sleep trait, to their offspring. - Source: parent.

While I agree that sounds like a plausible theory (and sort of reminds of the domestication theory... ie cute animals get more parental and human focus) but there are just too many animals that barely take care of their young that still sleep.... for example sharks sleep.

Also, why would we keep on sleeping into adulthood if it was such a massive risk and waste of resources? It doesn't make sense that adults also sleep when they're "getting a break from their kids"

> forcibly keep an animal awake for long enough and you will kill it. The same almost certainly applies to humans.

There was a PBS Nova episode on sleep many years ago. It showed people who never sleep. They are rare but they exist and have been studied extensively. They do have down-time where they rest for hours, but they never fall asleep. If this was any other show than Nova I'd call bullshit.

I believe one explanation for this may be that even if you think you've been up for a very long time, you will still have unknowingly experienced some microsleep states.

Animals in studies seem to die after around 30 days (which I think is longer than any scientifically verified claim made by a human?), and also may even have microsleeps prevented, depending on the experiment.

Humans may also have some rudimentary system of "local sleep". Some other animals have much more sophisticated systems. Basically, parts of your brain might "sleep" and become inactive temporarily, while others are still functioning. I don't think there's any hard evidence of this in humans yet, though.

Also, the longer you stay up, the less aware and cognizant you are, so you might even doze off for an hour or so and not actually realize or remember it. I imagine the people Nova reported on were studied and confirmed not to have actually napped, but it probably does apply to many people who give personal anecdotes of staying up for more than a week.

The only cases of a human truly "never sleeping" are from fatal familial insomnia, I think, and it is as fatal as it sounds like.

So, I suspect if an experiment really kept a human up for a month and prevented microsleep, they'd also die. Even if they didn't die as a direct result, their immune system would be so compromised that they'd probably die from a pathogen pretty quickly.

You're sure they weren't talking about people on polyphasic sleep schedules?

Also, fatal familial insomnia is a thing, so we have a pretty good idea of what happens when people are deprived of sleep for too long.

FFI is a bad model for long term sleep deprivation since massive neuronal loss is also involved.

Just asking: do we know that massive neuronal loss does not occur with "normal" long term sleep deprivation?

Are there any decent sources for Nova episodes online? It's been years since I watched any of them but I'd love to dive back in.

Um, the Nova website, http://www.pbs.org/wgbh/nova ? It has episodes as well as shorter videos.

A related question: why do some (actually a lot of) people grind their teeth during sleep?

According to wikipedia: The ICSD-R states that 85–90% of the general population grind their teeth to a degree at some point during their life, although only 5% will develop a clinical condition.

I'm not sure, but anecdotally, one person I know said that it had been an issue, until he started limiting how much liquid he drank starting 3-4 hrs before bed.

Have their been any notable causes? My money is on stress.

Not just

Of course, we sleep because we evolved from other forms that already slept!

The understanding of sleep isn't complete without taking into account why animals sleep.

Sleep could have gotten more complicated during evolution; acquiring new functions.

Running repeated passes of lossy compression.

My guess: predators hunt at night. The quieter you are, the more likely you are to survive another night.

Our ancestors (primal mammals) were nocturnal. They slept during the day.

I never think about this question, now I am thinking it, but the question may be that I cannot think it all the time without doing anything else, Ok, let's go to sleep for some rest.

"Because we get tired." is what I've always heard from people that aren't researching it.

Define tired.


Apt, but doesn't that describe most of what our bodies does even when we're awake?

I have another idea why we sleep: the reason lays in physics, it happens because the sun is not shining all the day. So the living creatures had to create a mechanism to be more active while daylight. That is why all “garbage collection” activities were moved from daylight activity to the night.

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