Why do our eyes close when we sleep? Is it to relax our eye muscles? How can it be explained from an evolutionary point of view?
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1$\begingroup$ The human optic nerve connecting an eye to the brain has about 1,000,000 or 10^6 fibres, each with a response time of about 1/100th of a second. In computer terms that accommodates a bandwidth of about 100 Megabits per second. We are awake in general for about 16 hours per day which is 57,600 seconds, so the brain receives from the eyes about 10 trillion or 10^13 informations bits per day. Under extreme conditions the the human brain can stay awake for longer than this, but its performance degrades significantly and becomes unusable at a week (168 hours) of wakefulness. During a week of wakefu $\endgroup$– Don Herbison-EvansCommented May 27, 2017 at 12:10
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3 Answers
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Our eyelids close when we sleep probably for the obvious reason that it prevents the sclera and cornea from drying out, becoming accidentally scratched (such as blowing dust) and allowing oxygen diffusion from the inside of the eyelid (to the sclera and cornea). Fragile corneas are a requirement for our vision. Thick corneas are much less fragile but then attenuates more light.
From terdon:
1) blinking requires muscle movement which sort of defeats the purpose of resting
2) closed eyelids offer much better protection than periodic blinking
3) if you can blink, you have eyelids.
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3$\begingroup$ @speedoheck 1) blinking requires muscle movement which sort of defeats the purpose of resting 2) closed eyelids offer much better protection than periodic blinking 3) if you can blink, you have eyelids. Why bother flapping the garage door up and down every few seconds when you could just close it and be done with it? $\endgroup$– terdonCommented May 13, 2014 at 2:06
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2$\begingroup$ @speedoheck Because you need to be able to see of course! Take the car out if you like. In sleep, there is no such necessity. Also, we get a lot of visual cues from light and that affects the concentrations of certain hormones. I would guess that is also an important reason why our eyes need to be closed. $\endgroup$– terdonCommented May 13, 2014 at 2:15
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$\begingroup$ @terdon. if evolution gave us the protection of eyelids during sleep, why did it give us fragile corneas $\endgroup$ Commented May 13, 2014 at 2:18
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3$\begingroup$ Fragile corneas are a requirement for our vision. Thick corneas are much less fragile but then attenuates more light. $\endgroup$– user560Commented May 13, 2014 at 2:26
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$\begingroup$ Perhaps all these comments should be added to the answer. $\endgroup$ Commented May 14, 2014 at 20:02
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There's definitely something to the fact that eyes work better when they are closed periodically and don't work well when they are open constantly, but there is another angle to this topic.
I think its worth adding that not all animals sleep with their eyes closed.
This is because some animals sleep with only half of their brain at once. Ducks, some birds, aquatic mammals like whales dolphis and seals engage Unihemispheric sleep. In Unihemispheric sleep, half the brain is asleep and the other is awake. Not necessarily fully awake, but enough that one eye is open and watching for predators or other threats.
One might ask - didn't we all have predators and threats? Why don't we all sleep half the brain at a time? The answer is not clear, but it appears that unihemispheric sleep is not a trait that is maintained easily. Very few animals really have the ability. As such there must be some advantages physiologically to closing both eyes at once and experiencing total sleep. Once the threat is not so great from being sleeping prey, animals start to close both eyes pretty quickly.
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1$\begingroup$ is it necessarily the case that full sleep confers an advantage? Couldn't it also be true that unihemispheric sleep is advantageous but the requires very complex adaptations that have simple only happened in a few species? Or is the likelihood/difficulty of a mutation taken into account when biologists use the term "advantage?" $\endgroup$ Commented May 27, 2017 at 12:22
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1$\begingroup$ Great comment. I think its the case the unihemispheric sleep is definitely an advantage! Keeping one eye open must be very helpful. You have hit the nail on the head. the advantage of an adaptation must be large enough to eliminate the mutants that don't have it efficiently. that is pretty much how I understand 'advantage': 'advantage' means an adaptation will stay around... $\endgroup$– shigetaCommented Jun 8, 2017 at 3:18
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1$\begingroup$ Through psychology, It shows that proper sleep aids in memory retention and learning. It could be possible that evolution led some animals/organisms to better retain information by allowing for full sleep compared to unihemispheric sleep. Just food for thought. $\endgroup$ Commented Oct 20, 2017 at 5:26
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Tears contain an important hormone that kills bacteria. Closing eyelids during sleep allows a prolonged contact between the cornea and tears killing resistant bacteria in the process. The action of the tears during blinking is not sufficient to completely get rid of the foreign bodies in the eyes. Closing eyelids gives the tears a capacity to accumulate forming a film of moisture above the cornea and acqeous humour. This has many important uses. It allows dust particles to dissolve and be eradicated when the tears flow out of the eyelids. This partially explains the residue that is seen on the cheeks when people wake up in the morning in addition, of course, to the sodium and potassium salts in the tears. The other importance of the film of tears accumulated on the eye is to help keep the acqeous humour moist. This has a great effect on how the lens receive light and also on the overall refractive index of the eye as system.
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1$\begingroup$ Any references to add to allow for background reading? $\endgroup$– AliceD ♦Commented Feb 1, 2016 at 0:03
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$\begingroup$ "an important hormone that kills bacteria" ... which is? $\endgroup$– rg255Commented Feb 1, 2016 at 13:37
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$\begingroup$ @AliceD One reputable source is: "Antimicrobial Compounds in Tears" (Jul 20 2013), by Alison McDermott, published on the US National Library of Medicine website. There are similar articles about lysozyme elsewhere. $\endgroup$– RobCommented Apr 3, 2019 at 19:46
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$\begingroup$ @Rob - in that source I've searched for hormone and it's not mentioned anywhere in the paper. Please adapt answer and include the reference in the answer and not in the comments $\endgroup$– AliceD ♦Commented Apr 4, 2019 at 7:04
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$\begingroup$ I agree that hormone isn't there (or in relevant information elsewhere). Pride Chigs only used the word once in their answer. Whether they found something that I wasn't able to, confused hormone with enzyme, or confused the types of tears their properties and function, is unknown to me and will have to await a reply from the OP. I'm reluctant to make edits where the meaning (premise) of the question or answer is altered as reviewers are keen to reject such edits, and then I must successfully appeal (increasing the effort). So I suggested an improvemnt $\endgroup$– RobCommented Apr 4, 2019 at 12:52