The Evolution Of Sleep

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.

I'm not sure if this applies generally to predator/prey relationships (although I suspect it does), but since you're talking cheetahs, grazers on the African plains tend to sleep much, much less than the hunters. Whereas hunters will happily spend more than half the day asleep (lions average something silly like 16 hours a day), antelope only sleep a few hours, and this they do in short naps, whilst standing up, and are easily startled awake by loud noises.I imagine the antelope are pushing the limits of what you can get away with; and it probably makes more sense from an energy efficiency point of view for the lions to not directly compete on the sleep front, instead conserving energy for as long as possible between meals. If they catch sufficient meals in their few hours of wakefulness anyway, why increase your energy demands by only sleeping a quarter as often?Edited by caffeine, : No reason given.

Where did we establish that it evolved independently in deuterostomes and protostomes? Catholic Scientist threw it out there, but I can't find any evidence anywhere for the idea. If sleep is so widespread even in quite simple animals, a single origin seems the default assumption to me.

I don't think the amount of sleep is that arbitrary. It's related to ecological factors - how much sleep you can get away with - and metabolic factors - animals with a slower basal metabolic rate sleep less. Elephants have a very low basal metabolic rate, and need to spend a lot of time travelling great distances to get a lot of food, so need very little sleep. Rats have a high metabolic rate, and tend to sleep safely tucked away in burrows where they're mostly protected from danger, so can afford to sleep for a long time. Antelope have a higher metabolic rate than elephants, but only sleep a couple hours more because of the ecological constraint of having to be alert for predators most of the day.Here's something weird I just discovered whilst reading the wikipedia aricle about sleep, which clearly shows that there's more going on than simply energy conservation. Hibernating animals need to sleep. To do so, they have to come out of their hibernating state, thus increasing their energy consumption.My unevidenced speculation is that dreams may be hallmarks of something important. Perhaps one of the functions of sleep is to do something with the brain which cannot be done in a waking (or hibernating) state, and dreams are the byproduct of whatever this process is.Edited by caffeine, : Because I acceidentally wrote that elephants need lots of sleep, when I meant very little sleep.

But animals still seem to go for as much as their lifestyle and safety would permit, so whatever the reason is that we sleep, it seems to be pretty damn important. You call that 'a bit' vague? It couldn't be any less precise if I tried. You could try looking at this page, all about dream research from UC Santa Cruz. The brief glance I've had seems to argue strongly against my vague speculation though. For starters, they claim that you don't start to dream until the age of about 5 or 6, and thus animals with less detailed cognitive worlds probably don't dream. I'll have more of a read later.