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The process of sleep seems to be very disadvantageous to an organism as it is extremely vulnerable to predation for several hours at a time. Why is sleep necessary in so many animals? What advantage did it give the individuals that evolved to have it as an adaptation? When and how did it likely occur in the evolutionary path of animals?

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e.g. dolphins can't afford to lose full consciousness when they rest, otherwise they would drown. So their mammalian brains have evolved in such way that only one half of the brain sleeps at a time. –  Oliver_C Jan 30 '12 at 10:46

3 Answers 3

This good non-scholarly article covers some of the usual advantages (rest/regeneration).

One of the research papers they mentioned (they linked to press release) was Conservation of Sleep: Insights from Non-Mammalian Model Systems by John E. Zimmerman, Ph.D.; Trends Neurosci. 2008 July; 31(7): 371–376. Published online 2008 June 5. doi: 10.1016/j.tins.2008.05.001; NIHMSID: NIHMS230885. To quote from the press release:

Because the time of lethargus coincides with a time in the round worms’ life cycle when synaptic changes occur in the nervous system, they propose that sleep is a state required for nervous system plasticity. In other words, in order for the nervous system to grow and change, there must be down time of active behavior. Other researchers at Penn have shown that, in mammals, synaptic changes occur during sleep and that deprivation of sleep results in a disruption of these synaptic changes.

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I found this paper by Benington and Heller that expands on the previously mentioned theory of sleep as a mechanism to renew metabolism. They hypothesise that sleep is necessary to replenish glycogen stores (mainly within astrocytes) in the brain. These stores are normally used to supplement blood glucose due to the high energy demands of the brain.

It is suggested that this may also result in the manifestation of feeling sleepy as a consequence of the exhaustion of glycogen supplies in specific small areas of the brain. Brief and localised depletion of glycogen stores mean that cells are operating with less energy than they normally have to work with. This causes an increase in synthesis of adenosine from the breakdown of AMP. The paper maintains that the increased levels of adenosine are detected by adenosine receptors which then triggers or increases (as seen on EEG scans) the feeling of the need for sleep:

Flowchart of the relation of glycogen to sleep need and replenishment

In NREM sleep (which may lend some support to the theory as NREM sleep accounts for 80% of sleep and is most physiologically different to waking), this glycogen is most efficiently replaced. During NREM sleep the release of glycogenolysis inducing neurotransmitters is reduced, allowing glycogen-synthase to predominate and glycogen levels to be restored. However, these same neurotransmitters are key in the processing sensory stimuli (by tonically depolarising neurons in the sensory cortex). Therefore glycogen replenishment will always be associated with a (strongly) reduced response to stimuli.

This leads to finally answering the question as to why sleep has an evolutionary advantage, which I will quote verbatim to maintain the authors momentum:

Glycogen replenishment during waking would be maladaptive because it would impair the organism's ability to to process and respond to sensory stimuli. Sleep has therefore evolved as a state where animals retreat to a safe environment, behaviour is suppressed and glycogen stores are replenished.

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So is it just a coincidence that the metabolic pathway of glycogen synthesis in brain evolved to be regulated by neurotransmitters? If it had evolved independently - we wouldn't need any sleep? –  Marta Cz-C Jan 29 '12 at 13:51
    
@MartaCz-C that's a really good point, and if I'm honest I've struggled to find an answer in literature for you. However it could be said that as the glycogen reserves are in essence a backup power supply, even if neurotransmitters were not involved such a supply is not likely to be replenished without a greatly reduced energy demand (i.e. in sleep) as otherwise any glucose that arrived would be quickly used rather than stored in glycogenesis. It could be an adaption rather than a co-incidence if this is the case - forcing an organism to take a period of rest to rebuild these supplies. –  Rory M Jan 29 '12 at 21:22

From what I've learned, there are two theories to answer that question:

  1. Restoration - the body needs to rest in order to renew its metabolism (if an animal is active 24/7, it will constantly use up a lot energy and metabolism). It has been shown that mice that have gone a while without sleeping have a compromised immune system.

  2. Preservation - sleep is assumed to confer a survival advantage. Night time is a dangerous time, so sleep sort of 'forces' an animal to lay low for a few hours.

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Your point #2 says exactly the opposite of the question's premise. Would you care to expand a little bit (e.g. give some references)? –  nico Dec 24 '11 at 20:39
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Point 1 seems to be a circular argument (or flat out wrong): if the animal weren’t sleeping it could hunt / forage for food. Even in sleep, we expend some energy so this calculation will always come out favourably on the side of staying awake. –  Konrad Rudolph Dec 29 '11 at 9:46
    
@KonradRudolph - the body always expends some energy, but the energy the body uses while sleeping is a fraction of the energy it takes to stay awake/hunt/etc. –  Innab Dec 30 '11 at 11:46
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@Innab It’s irrelevant that the energy expense is only a fraction. The important thing is that there is any at all. Do the math, it will never be beneficial to rest instead spending the same time hunting, because the net energy balance of hunting is positive (i.e. energy take-in rather than expense), so in sum, resting will always spend more energy than hunting. –  Konrad Rudolph Dec 30 '11 at 12:42
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@KonradRudolph surely that is only the case if the hunt is successful - therefore sleeping can be less disadvantageous in terms of energy expenditure if the odds are heavily stacked against successful predation at a certain time of day. –  Rory M Jan 29 '12 at 12:16

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