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.

https://www.ncbi.nlm.nih.gov/pubmed/24822040

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.

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)

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.

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.

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

http://www.ted.com/talks/jeff_iliff_one_more_reason_to_get_a...

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.

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

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?

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.

> 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.

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.

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.

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.

Maintenance.

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.

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.

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.

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?