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.

Thread copied here from the The Evolution Of Sleep thread in the Proposed New Topics forum.

Here's some of my musings about sleep:When it gets dark (for diurnal animals) it is very dangerous to be stumbling around in the darkness. Sleep could be really good way to immobilise the organism and conserve as much energy as possible.We we sleep we experience sleep paralysis. If we did not we would act out our dreams. This means that when we sleep our brain is as active as when awake and something about that activity is necessary for the correct working of the brain.There is some suggestion that brains 'de frag' when we sleep; ordering our files in a more efficient way. I personally like this: anecdotally I've spent ages on a tricky computer game only to find it easy when I've slept on it.The various stages we go through when we sleep is quite plastic, we can condense two to three cycles into aboutvfour hours if we need to. But the fact we can't reduce it to zero and the way some animals sleep different hemispheres of the brain separately is a strong indication that sleep is biological necessary. After all, not sleeping for long enough leads to irritability, paranoia, hallucination and death.I 'reckon' that having a complex nervous system means it 'run hot' and needs to have a refractory period where it de frags itself and this function has been co opted by organisms to conserve energy and/or stay safe during none feeding or mating times.I've not touched on dreams but they are fascinating, too.

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.

Don't most animals have a "night watchman" so to speak? A member of the group that watches out for danger?

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.

As far as I can recall babies do most of their growing and sundary protien syntheses when they ae asleep.As far as I know all animals need sleep. I 'reckon' it did not evolve specifically but was a by product of the advent of the nervous system.

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)

It gets dark anyways, so you might as well just wait for when you can see. Being able to shut down while you're waiting and avoiding unnecessarily spending energy would be beneficial and so would the rebuilding aspects. Pretty much every animal sleeps in some way, so it’s been important since early on. Fish and reptiles have a more primitive sleep state than birds and mammals. And even invertebrates have a shutdown mode. But I don’t think non-animals sleep. I guess that as you get more complex systems within organisms, the more they need to regulate them. I doubt it from our current state, but I suppose it would have been possible. Although, the energy input would have to be a lot higher so that might be preventative. Don’t forget that predator-prey relationships have to form a balance, or the preds will eat all their food, so the vulnerability thing would kind of work itself out. Well this is all just me speculating.

Given that virtually every animal sleeps i think sleep must somehow be necessary for normal brain function to work.fun fact:
Dolphins sleep with only half a brain at a time because they have to be constantly concision so they do not drown.

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.

Well, it could be... If the circadian rhythm was already a part of their evolution before they got to the poles/caves. If we're going back that far, I'd prolly bet that sleep evolved independently between Protostomes and Deuterostomes. Protosomes splits into antropods, flat worms, and mollusks (etc) while Dueterostomes splits into urchins, round worms and vertebrates (etc). I think that there's some disparity between the sleep that these two groups exhibit, so that could suggests its unrelated. I don't know, but there's also a lot of "rebuilding" that goes on during sleep, which would still count as 'activity' even though its helping. Its still gonna require energy, but you're not doing all that other activity as well. If you didn't sleep, then you'd have to be rebuilding while you're doing your normal activities, and then all that together would require that much more energy. Adaptations to sleep that arrise after sleep has already evolved isn't gonna help us much in figuring out how sleep evolved in the beginning. To contrast: How much energy does a brown bear save during its hybernation? That's not really what I was thinking... Preds that took too much advantage of sleeping prey wouldn't have survived, so we'd just be left with the ones that didn't. I see it as more of a balancing act than an arms race. With aliens, having their own circadian rhythm, the sky's the limit...But sleep seems like a good idea to me. Everything degrades and nothing can run forever, it just makes sense to take a break every so often. Would you prefer to never have to sleep but then just have a lifespan that was 30% shorter? Would you sleep more for a longer life?

Well, our ancestry with squid is a very long time ago, but are you sure they don't sleep? I had a look and found this article on Discover about cephalopod intelligence, which has this to say about sleep:

quote:

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Meanwhile, Anderson has been investigating another phenomenon little-noted in invertebrates: sleep. Until recently, only vertebrates were believed to sleep in the full metabolic sense. But Anderson has observed that octopuses, ordinarily hypervigilant, may sleep deeply. Their eyes glaze over, their breathing turns slow and shallow, they don't respond to light taps, and a male will let his delicate ligulathe sex organ at the tip of one armdangle perilously.

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Stephen Duntley, a sleep specialist at Washington University Medical School in St. Louis, has videotaped similar slumber in cuttlefish, with a twist: Sleeping cuttlefish lie still, their skin a dull brown, for 10- to 15-minute stretches, then flash bold colored patterns and twitch their tentacles for briefer intervals. After viewing Duntley's footage, Anderson suggests the cuttlefish might merely be waking to check for threats. But Duntley says the cycling resembles the rapid eye movement sleep of birds and mammals, when humans dream. If invertebrates undergo a similar cycle, Duntley argues, it would affirm "that REM sleep is very important to learning." Would it also suggest that cuttlefish and octopuses dream? "That's the ultimate question," Duntley responds.

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20% of energy consumption seems to be the most commonly cited figure.

It is true that the human brain is musts as active as when it is awake so I don't see sleep as an energy saver: the energy saved comes from over all inactivity.Swimming dolphins and flying swifts does so not wholes asleep but by shutting down one hemispher of their brains so they are never unconscious.I would imagine this could be the same in Cephalopoda, too. I've octopuses go into what looks like a refractory period and always assumed they were having a kip.I would hazard that organisms probably could exist that do not need sleep but they would be out competed by ones that do sleep. That's why we don't see any and organisms that would logically be better off without it (dolphins and swifts) not needing to sleep but have convoluted work arounds (sleeping one hemisphere at a time).

Forgive my pedantry: bears go into torpor, rather than hibernation.