Something has perplexed me for some time now. Why do humans (and other animals) need sleep? And as perhaps a second question, Why do we dream?

Oh, I’ve heard my share of explanations over the years. When I was in high school, one student read an article that said it (i.e., dreams) is for our “psychological equilibrium”. But for some reason I doubt that. Also, I have heard it is for our brain to process memory. But if that were so, wouldn’t dreams all be about the day before? I don’t know about you all, but that is rarely the case for me. In fact, some of my dreams are quite unique in that they have absolutely nothing to do with my waking world. I assume it is the same for all of you.

In any event, I was just wondering what the current theory is about sleep/dreams. (One ironic development is Provigil, a drug that may make sleep obsolete. Is that possible?)


One theory of sleep is that it was evolutionary favored because it stops you from roaming around at night and getting killed.

The most recent theory I heard of dreaming is that we mentally train during nights. That’s why dreams contain mainly stuff like falling, escaping etc, mainly difficult or important stuff. Also, we dream of tasks we have been involved in a lot lately.

I haven’t thought about this in detail, but do we (i.e. mammals) come originally from cold-blooded species (reptiles, amphibians)? For those, sleep is easy to explain: lacking a sufficient heat source during the night, they conserve energy by shutting down, essentially. It could then be that sleep acquired other advantages gradually, perhaps linked to the maintenance of a growing cerebellum, which led to it not being abandoned even as we became endothermic.

As for dreams, I’m not sure there’s any conclusive evidence on their being more than interpretations fostered upon essentially random neural noise – sort of the internal equivalent to seeing Jesus on a piece of cheese sandwich. That they do seem relevant to recent events can be easily explained by priming: what you have grounds to expect, you are more likely to see. Basically, sensing is a hypothesis-elimination programme: rather than mining the data you receive for whatever could be in there, which would be computationally rather challenging, you go in with a set of preconceptions about what you expect to find in the data, and gradually dispel these expectations (it’s a rather scientific process, actually). Those expectations are built from experience. When you sleep, you suffer from a dearth of data, and in order to make sense of what little you do receive (mostly noise), the threshold for hypothesis falsification is lowered, and thus, you become more likely to ‘see things’ in the static. This also accounts for the strange causal leaps in dreams, as there is no actual, consistent reality behind those ‘fake’ perceptions.

The problem with this is that it’s perfectly possible for an animal to remain motionless without losing consciousness. Instead sleep actually knocks us out, and if the night is so dangerous that simply wandering around is likely to result in death, it hardly seems to be a good time for enforced unconsciousness. If you have ever walked up on a sleeping bird or animal you will know just how dangerous sleep is to animals in the wild.

The problem with this explanation is that we don’t dream when we sleep. In an 8 hour sleep period we only dream for about an hour. We very specifically and deliberately enter periods of REM sleep wherein we dream.

What this demonstrates is that it is perfectly possible to sleep without dreaming, since most of our sleep is dreamless. So that puts the kibosh on the idea that dreams are just a side effect of sensory deprivation. There is no evidence that we receive any more stimulation during non-REM sleep than during REM sleep.

We can add to that the ability of people to sleep walk, a situation where all normal sensory inputs are perfectly available and able to be processed, but where dreaming continues and simply incorporates those inputs. The dream clearly isn’t the result of lack of stimuli since it subverts those stimuli and overrides them.

Moreover since we have evolved a deliberate dream sleep state that we must enter for effective recuperation, and since in that state we are highly susceptible to being ambushed, it seems fairly obvious that it has some sort of evolutionary advantage.

What it is remains open to debate.

That’s true. One might amend my WAG by stipulating that only specific phases of sleep possess the right characteristics to be conducive to such dream-production – one might be too deeply asleep to even care about making sense of the noise, or something like that – but the crucial question then seems to be whether lack of dream sleep has any deleterious consequences, i.e. if dream sleep is essential or might be accidental, which I don’t know.

There’s no doubt about that, it’s called REM sleep, and the brain deliberately enters into that state. It’s not accidental.

Once again though, we have the problem of sleepwalking, where people can walk, operate machinery, turn on the lights because it is too dark, carry on conversations and do all sorts of things that prove that they have the ability to make sense of stimuli, including spoken and written language. Yet all this is occurring during REM sleep.

There’s been a lot of work done on this, particularly associated with sleep apnea research. The basic finding is that lack of REM sleep is almost as bad as a total lack of sleep.

The brain usually enters REM sleep after about an hour of normal sleep, so researcher can wake volunteers as soon as they start to enter REM sleep, and then let them go back to sleep.

What you find is that even though the volunteer only loses about 2 hours sleep a night, they perform as badly the following day as people who only got two hours of sleep all night. Control subjects who were woken just as frequently, but always woken *after *an REM sleep cycle, performed as well as controls who had an undisturbed 8 hours sleep.

Even more interesting is that the time delay to REM sleep declines. A normal person goes into REM after about an hour. After being deprived of the first couple of cycles the time declines to 50 minutes, then 40 etc. until after couple of days the victims start going into REM sleep almost immediately.

What this all suggests is that dreaming plays an essential role in the recuperative role of stress, and that the brain has evolved to ensure that it occurs.

The big mystery remains why we dream, or for that matter why we sleep at all. It all seems pointless form a physiological POV, yet clearly it isn’t.

That’s interesting. Still, it seems to remain open whether the dreaming itself is necessary, or just accidental to whatever maintenance processes (or whatever) occur during REM sleep.

That’s true enough. The general assumption though is that dreaming is the reason for REM sleep, rather than a side effect. But as you note, it’s impossible at this stage to say for certain.

Sleep and the need for it is an interesting subject. I have sleep apnea. Back before we had it diagnosed and I got my CPAP, I used to hallucinate because of lack of REM sleep. Middle of the day, I’m dragging myself through classes, constantly in danger of dozing off into an inevitable if unprofitable (in terms of actual rest) fugue state, and I’d suddenly realize that I was seeing and hearing people who weren’t there. My brain was trying to dream whenever it could slip in an image here or there. I still view that part of my life as a very long nightmare. I used to fall asleep standing up!

When I went in for my sleep study, they went three hours of me trying to sleep “normally”. I remember that nights would last FOREVER back then, because I’d wake up constantly as my breathing stopped and it would just stretch the night out to the point where I was usually happier to just stay up. After three hours of this sleepwakesleepwakesleepwake, they fitted me with a CPAP mask and I tried again. They woke me up three hours later and I felt like I’d just closed my eyes.

The EEG of the night showed that I NEVER entered REM sleep during the three hours without the CPAP. I don’t think I ever slipped deeper than Stage I, because I stopped breathing 190 times in those 180 minutes! After they put the mask on me, though, I not only immediately fell asleep, but almost immediately dropped into REM. And I stayed in REM for the entire 3 hours. The EEG was just a solid black smudge all the way across.

Anytime I start to grumble about how my CPAP mask is starting to lose its seal or rubbing my nose or whatever, I remember that EEG and remind myself that machine changed my life.

Anyone who can definitively answer these questions will likely win a Nobel Prize. The purpose of sleep and dreams is, in my opinion, one of the biggest unsolved mysteries in biology today. Blake has already explained problems with several of the conventional explanations.

One of the more remarkable aspects is that sleep seems to be almost as vital as food. Rats deprived of sleep for several weeks will eventually die. Yet we don’t understand what exactly kills them.

It may be mentioned that some animals which need to move continuously (oceanic birds like albatrosses, that may remain on the wing for years at a time, and dolphins, which need to surface to breath) have developed the ability to sleep with half their brains at a time, while the other half maintains control over movement. Considering the disadvantages of being unconscious for long periods of time, it’s surprising that if this is an option more animals haven’t taken advantage of it.