June 26, 2007 3:01 AM   Subscribe

Why do we sleep?
posted by chuckdarwin to Science & Nature (23 answers total)

This post was deleted for the following reason: "Whence the phenomenon of sleep" might be a pretty answerable question, but given the presentation, this seems like let's-talk-about-sleep chatfilter.

In humans, other mammals, and many other animals that have been studied - such as fish, birds, mice, ants, and fruit-flies - regular sleep is necessary for survival. Why it’s necessary, no–one knows, but the suggested theories are to be read in the ‘Function’ section of that page.
posted by Aidan Kehoe at 3:05 AM on June 26, 2007

I venture a guess that sleep is a function of metabolism. And because it is late and I am tired.
posted by hortense at 3:30 AM on June 26, 2007

WNYC's Radiolab did a whole show on why we sleep about a month ago.
posted by mdonley at 3:38 AM on June 26, 2007 [1 favorite]

Short answer: No one knows for sure, but there are lots of theories.
posted by Brandon Blatcher at 3:42 AM on June 26, 2007

Why do we sleep?
Because we get tired?

I always thought it was to let your body rest and repair itself folllowing a hard days work.
posted by Nugget at 4:07 AM on June 26, 2007

Brain/body recharging or maintenance.

Lack of sleep causes people to operate at diminished capacity, whereas a good night's sleep can really recharge your mind and body.

But why sleep? It's dangerous for an animal to be so unprotected for long periods, right? It's not the sort of thing an intelligent designer would put into us, right? I'm guessing our bodies developed the best solution it could. Perhaps the need for sleep and the danger it presents also causes us to be social. It's better if we stick together so that somebody might be awake or wake up quickly enough to alert the tribe that a lion is here and boy is hungry.

I suspect that moving and interacting with the world consumes a lot of energy and the brain has split on how to handle all the sensory input. There's the here and now, where the brain can actively sort though the data and make changes as needed right then and there to ensure continued survival.

But sleep allows the brain to sort through all that data in a more careful fashion without having to worry about sorting new data at the same time, so it has more time to "think". Studies have shown that the brain is pretty active while we're sleeping, sometimes even more active when we're awake. Something is going on up there.

Giving the body physical rest probably allows it to divert energy to repair functions as opposed to operating the body.
posted by Brandon Blatcher at 4:22 AM on June 26, 2007

and for married people, it's quiet time away from the spouse.
posted by Brandon Blatcher at 4:23 AM on June 26, 2007 [3 favorites]

I have read recently that one of the main working hypothesis for sleep is about processing memories.

But I can't find where I read it, so maybe I need to go back to sleep a little more.
posted by bru at 5:35 AM on June 26, 2007

Imagine a society where power tends to concentrate through positive feedback - the rich get richer and the poor get poorer, the well off tend to band together to consolidate their strength etc. However, every so often a revolution happens - perhaps triggered by a poor harvest - and the existing power structure gets weakened or perhaps even completely reorganized. These periodic revolutions allow the society to remain dynamic and creative rather than be completely locked into fixed patterns.

I think this is what the brain is like. Triggering the revolution while the body is immobile (e.g. by modulating the excitability of the neurons of the brain) ensures that it does not produce pathological behavior when it happens (e.g. epilepsy).

What this is really about is using the intrinsic properties of the elements of the system to produce autonomous homeostatic regulation. This is a much more evolutionarily accessible strategy than developing a specialized all-seeing central controller, which far outweighs the obvious disadvantage that the brain has to go offline every night in order for it to work.

Read Tononi and Cirelli's work on sleep and synaptic homeostasis if you're interested in a much more detailed version of this line of thinking, backed up by experimental evidence.
posted by teleskiving at 5:38 AM on June 26, 2007

Sleep is to the brain as defragging is to a hard drive.

Completely untechnical and without factual basis, but it has the ring of truth.
posted by squidlarkin at 5:41 AM on June 26, 2007

evolutionarily speaking, the reason isn't positive, it's negative. in other words, it's not why we sleep, but why we don't sleep more or less than we do.

and the answer is: because it doesn't kill us before we can reproduce.

beyond that, periodic sleep probably conferred some advantage onto our distant ancestors--perhaps it made him smarter, or more biologically efficient, or whatever. whatever it was, our ancestor didn't sleep so much that he was vulnerable to predators, but slept just enough to gain whatever benefits sleep conferred, and that made him the best at spreading his genetic material around.

that said, i'm going back to bed now. i'm tired.
posted by thinkingwoman at 5:43 AM on June 26, 2007

Look at, say, chimpanzees: they don't have to search for food constantly, they have access to mates within the social group, and they don't all need to be checking for predators all the time. Why would you not sleep? You get to reduce your metabolism, and give your body time to repair damage. It's highly adaptive.

So why can't we eliminate sleep? We were designed with sleep as a biological feature, and there's no evolutionary pressure to lose it.
posted by crocomancer at 5:50 AM on June 26, 2007

Or pretty much what ThinkingWoman just said. D'oh.
posted by crocomancer at 5:51 AM on June 26, 2007

As a piggyback clarification, I'd like to ask whether the reasons "we" (human beings) sleep differ from the reasons other animals sleep. If so, if our psychological processes are different enough from other animals' comparable processes, then we're talking about something that natural selection may be able to refine or eliminate, are we not? (This possibility requires a lot of time that our species may not have, but let's table that.)
posted by cgc373 at 5:54 AM on June 26, 2007

The evolutionary angle to thinkingwoman's post makes me think this is all vaguely eponysterical.
posted by tim_in_oz at 5:54 AM on June 26, 2007

I have read recently that one of the main working hypothesis for sleep is about processing memories.

I heard this too. The theory was that sleep was when the brain took the day’s short-term memories and sorted them into the long term memory. This is why dreams are often a mixture of the two.

I’m sure someone smarter than myself (99% of you) can poke plenty of holes in that theory, but it’s the best one I’ve hears so far. I especially like the way it explains dreams, which I’ve always felt meant absolutely nothing, just your brain dealing with garbage.
posted by bondcliff at 5:55 AM on June 26, 2007

It improves learning:

"Human [1–4] and animal [5–9] studies have revealed experience-dependent reactivations of regional cerebral activity during post-training sleep, in brain areas previously engaged in learning during wakefulness. Furthermore, in humans, post-training reactivations in hippocampal ensembles have been found to correlate with overnight improvement in performance in a spatial navigation task [2]. Likewise, local increases in slow-wave activity during sleep after learning correlate with improved performance in a motor adaptation task in the post-sleep period [4]. Experience-dependent reactivations of cerebral activity are hypothesized to reflect the offline processing of recent memories during sleep, which eventually leads to the plastic changes underlying memory consolidation and a subsequent improvement in performance [10,11]."
posted by jwells at 5:59 AM on June 26, 2007

Tonight on NPR.
posted by kittyprecious at 6:27 AM on June 26, 2007

and for married people, it's quiet time away from the spouse.

Unless the spouse snores.
posted by candyland at 6:45 AM on June 26, 2007

If so, if our psychological processes are different enough from other animals' comparable processes

Psychologically, yes. Genetically and metabolically, not so much. I'd think our genetics and metabolism necessitate the need for sleep than psychology.
posted by jmd82 at 6:47 AM on June 26, 2007

The best answer seems to be that we sleep in order to keep us out of trouble.

If animals are capable of getting in enough feeding and copulation done in 12 hours per day to stay alive and reproduce enough, then if they were active the other 12 then all that would result is a greater vunerability to predators and a higher death rate. Creatures which sleep half the time are less likely to die. (They also need less food, a distinct advantage.)

Evolution tends to operate on a "But it was just sitting there, so I used it" basis, and all the other stuff (preventive maintenance of the brain) associated with sleep probably came later.
posted by Steven C. Den Beste at 7:30 AM on June 26, 2007

"Why?" is an unanswerable existential dilemma. You should ask, instead, what benefits do humans receive from sleep? If humans—and indeed, other mammals—receive benefits, those that sleep an optimal amount may have an evolutionary advantage over those that do not. Asking that question may yield better answers.
posted by monju_bosatsu at 7:32 AM on June 26, 2007

Its a lot of "some people think" statements.

Some people think REM sleep helps long-term potentiation (forming relationships in your brain) during development, acting as a stand-in 'visual' afferent.

People know sleep deprivation affects neural plasticity, but that's just an observation; two competing hypotheses behind that observation are that sleep lets individual cells (glia, I would guess) store up enough energy to allow for plasticity, and that sleep deprivation puts a general stress on the body inhibits the function.

It goes on like that. What tends to happen is the neurologists look at it and ask, "why are the cells doing ___," and come up with a reason for it, and the psychologists ask, "why are thoughts/memories like ___," and come up with a reason for it, and they're not necessarily opposing but completely unrelated. Neurologists love to be able to point to a specific cell under a microscope and say, "look at this, it reacts __ to __," while psychologists seem to be much more open to "well, we observed __ in ___ people under ___ conditions, so we're linking these two things."

/End horrible generalizations that probably are not remotely true.
posted by devilsbrigade at 7:43 AM on June 26, 2007

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