The Evolution Of Sleep

How and why did sleep evolve? Which forms of life require sleep and which don't? What does this tell us about sleep and how it evolved? Is it possible for complex life to evolve that doesn't require sleep?It has just occurred to me what a strange thing sleep is in a evolutionary context. A state of high vulnerability that has evolved. Why? Is there no way round it?I have no answers or strong opinions of my own. I am just intrigued as to the purpose, need and hence evolution of sleep.

You've obviously focused on human sleep. And that's all very on topic and fine. But from what little googling I have done on this it appears that all sorts of life on Earth sleeps. Not just mammals and reptiles but fish and even invertebrates (not sure about insects).So at what point did sleep evolve and why?Is it even possible for evolved complex lifeforms to exist without sleep?If we meet intelligent aliens one day will they also need to sleep?I have no idea. But insights from anyone who knows anything about this at all much appreciated. Indeed. If this thread catches on I'm sure we'll get onto dreams. I hope so.

In social animals very probably. But it doesn't seem to be prerequisite for sleep to be needed.I have just come across the notion of "unihemispheric sleep" where one half of the brain sleeps whilst the other half stays alert for threat detection etc. Apparently some birds and aquatic mammals utilise this method of sleeping. Fascinating.Here is a wiki link on non-human sleep: More generally: LinkSo sleep seems to be a very early developed, and hence widespread, evolutionary phenomenon indeed.

More than plausible.So at what stage in the evolutionary development life did the nervous system evolve?Are there any examples of creatures which exist today and straddle the divide between having a nervous system and not (and thus possibly straddling the sleep/non-sleep divide)

The circadian rhythm does indeed seem to be a feature of most life on Earth. Including plants as well as animals. No doubt it is a contributing factor to the evolution of sleep. But creatures evolved for polar conditions (where "days" and "nights" can last months) and those that dwell in caves, underground or deep within the oceans where day/night is of much less consequence all seem to sleep as well. So it can't be the whole story. Perhaps unsurprisingly single celled organisms don't exhibit any signs of anything matching sleep. But nor (apparently) do squids and octopuses. How far do we have to go back to find a common ancestor with them I wonder? Isn't most of the energy humans consume taken up by the brain? And (as I understand it) the brain isn't really less active when we sleep. I might need to check my facts here. Also dolphins swim whilst asleep and birds fly!!! (mad!!). So how much energy is really being saved? In the evolutionary arms race wouldn't it be a case (if possible) ever less sleep and improved nocturnal abilities rather than everyone agreeing to take the night off? I am intrigued by the idea of intelligent evolved life that doesn't need to sleep. If it is possible then maybe one day we will meet some alien cousins who will consider our need to spend 30% of our life unconscious thoroughly and bewilderingly inefficient.If of course the squids and octopuses haven't taken over by then....Edited by Straggler, : No reason given.

Isn't this unihemspheric sleep a great idea?Imagine if humans did that! We'd be all emotional and unrestrained creativity half the time and logically minded pedants the rest In what way? Adapatability?

Interesting on the octopus. Given that even worms have been observed to sleep I was surprised to read that squid and octopus don't. But it seems they do after all. Yes - That is what I have seen too. But I don't think it uses any less when we sleep.

But that still wouldn't fully explain fish or deep sea dwellers would it? If that is correct - That sleep has evolved along two independent strands - It adds weight to the notion that it is something that most complex lifeforms will do. So - In the same way we might expect alien lifeforms to possess "eyes" of some sort (because they are bloody useful and have evolved independently in a number of forms) - It seems aliens might well sleep too. Bizzarre. OK. But let me put my question/line-of-thought another way. If we develop Artificial Intelligence would we expect to implement periods of sleep into our inventions? Kinda defrag time? But isn't that like saying that if Cheetahs got too fast they'd all die out from over-eating? Whilst it might be logically true the incremental nature of change means that it doesn't happen like that. The Cheetah gets a bit faster and then the antelope does too (or vice versa) and the whole thing is kept in balance through graduality. But the ultimate result is escalation. So why not the same with regard to preying on sleepers? Well forever is a long time. But given the ability of organisms to repair themselves I don't see why the limit has to be tens of years rather than much longer. Trees live for centuries. Why, in theory, couldn't brains? But I'm not sure why those have to be the choices.

That raises the question of what makes us tired. How do we know/recognise when we are tired? What triggers it? Is it some sort of chemical/hormone?Can we learn anything about the biological nature of sleep from studying narcoleptics and insomniacs?

But even when awake it is possible to have very vivid hallucinations and visions (I'm not knowingly prone to such things but I have experienced my share of recreational drugs). And if I close my eyes I can very clearly imagine the face of my son (for example) who I last saw a couple of hours ago.So I don't think such clarity is necessarily a feature of dreams specifically.

I agree. I'm just not convinced by GDR's idea that images within dreams are necessarily superior.I suspect the main difference is that when we dream it seems more real because we are not consciously aware that we are dreaming and that it isn't real.Or something like that.

Oh I don't know. It just occurred to me how odd sleep is in an evolutionary context. Apparently I find it more odd than most. Although some googling around suggests that the question of why exactly we sleep is at least deemed an interesting scientific question that we have yet to fully answer.The fact that all complex lifeforms do seem to sleep and that it seems to have evolved at least twice independently (to our knowledge) suggests that there is something rather fundamental about it.I suppose what I am asking (to put it into geek speak) is why it is that despite all our seeming biological refinements we are more like Windows operating systems than UNIX. Windows needs to rebooted all the blasted time whilst my record for a UNIX box is seven years and counting.But now I am just letting my work frustrations infringe on the far more important activity of internet foruming......

Fair point.In my own ignorance I took CS's speculations to be more authoritative than even he probably intended. I can't find any support for this claim either.And I agree that it seems more likely that the origin lies in a common ancestor to both. Which makes sleep something that arose very very very early in our evolutionary past.So where does that leave us on the 'Would evolved aliens also likely sleep?' question?

The apparent inefficiency of it. The vulnerable state it leaves us in. The fact that a whole mechanism for allowing our imagination to run wild (i.e. dreaming) whilst restricting us from acting out these fantasies (we apparently undergo a sort of mild induced form of paralysis whilst REM sleeping) has evolved. The fact that despite all of us all doing it (along with practically every other complex living thing on the planet) scientists still don't really understand why we do it or how exactly it works. The strangeness of phenomena like sleepwalking (people have apparently sent emails and even cooked whilst asleep!!!), narcolepsy and insomnia. The lack of rhyme or reason in how much sleep is needed (a giraffe needs less than 2 hours of sleep a day and an elephant about 3 - Why? I would have thought size would make sleep more necessary). Etc. etc. etc.In general it just seems to be a very strange and interesting phenomenon. It just made me think - How the hell did that evolve? Apart from anything else with a five year old and one year old it is something which I am desperately short of and something which is much on my mind. LinkLinkEdited by Straggler, : No reason given.

Whilst the mind body problem is a fascinating topic in itself (maybe I'll start a thread at some point) but let's not explicitly go there here. But on that note....You must admit that physical things (e.g. drugs) can induce dream like states, hallucinations and all of the other things that you seem to consider non-physical. Isn't sleep (and dreaming) just bio-chemistry at the end of the day? Well if I have any say as the starter of the topic I would like to agree with you in a bid to keep those sharp eyed admins at bay.