Why do eyelids block out light?

[BoomerZ]

Eyelids do multiple things. Moisten eyes, remove objects in eyes, protect from damage from light or dust, the list goes on and on.

[LunarScyphozoa]

They’re also not 100% opaque, closing your eyes staring at a light and in a dark room will yield different results

[amymadison182]

Yeah it does, People don't see darkness with closed eyes, but they see a dark color which is the absence of light with closed eyes.

[DasIstWunderblyat]

Because if you don't occasionally block your vision by blocking the light, you can see it coming for you.
It does not like to be seen.

[LostKelp]

Also when our retinas are exposed to light we produce cortisol which wakes us up in the morning, so by lowering light levels we produce less cortisol which helps us sleep.

You may look at that and go but wait! Why do I wake up at 2pm if the sun has been up for 8 hours? Well although there is a correlation between cortisol levels and light, cortisol works more in a cyclical patten and you can change how that cycle acts with artificial light, caffeine and alarms. If we weren't living in a society that constantly has lights on, we would have 4-6 hours of sleep, wake up at midnight and wander around for a while and then have another 4-6 hours of sleep. This is called a biphasal sleeping pattern and is pretty universal among mammals.

Anyway a bit of a tangent and I might be a bit wrong about this but it's interesting none the less.

[BoomerZ]

Close but its way more complicated. This book is fantastic on the subject.

Why We Sleep: Unlocking the Power of Sleep and Dreams: Walker PhD, Matthew: 9781501144318: Amazon.com: Books

[LostKelp]

I'll check it out sometime

[river121693]

Are you asking "what is the purpose of eyelids blocking out light?" or "why is it that when I close my eye, everything is dark?" lol

[Ordinaryaverageguy]

There may be other benefits to having eyelids that cause eyes to stop receiving light but maybe those developed later. Eyelids are extensions of our skin & our skin is not really transparent, so maybe that's all there is to it. And that's also why we can detect strong light through our closed eyelids - because our skin is also not perfectly opaque.

[amymadison182]

Eyelids are nearly opaque, so people see a blackish color inside their eyelids, people say they see a very dark reddish-brown or a very very dark reddish-brown, other say that they see a very very dark gray, While people see a very very dark gray, whether in light or dark enviroments, so they commonly see a very very dark gray that's almost close to black.

[Ordinaryaverageguy]

Besides which, maybe this is different for people, but when I close my eyes I see all kinds of phosphenes. So, there's never any real darkness, even when I'm in the darkest of places.

[amymadison182]

It depends on all people whether they see a very very dark gray or very very dark reddish-brown with closed eyes, regardless.

[Legendofgenii]

I wonder if mermaids have eyelids.

[amymadison182]

They have eyelids, lol.

[NickRowler]

This has to do with eye fatigue.
People may notice it happening on sunny days or staring at a bright screen for too long.

Heck, the American flag optical illusion utilizes it.

[amymadison182]

Don't you mean an afterimage? all people don't see afterimages on sunny days, but they see afterimages when staring at a bright screen for little too long and I don't think afterimages has to do with eye fatigue.

[TAMAnnoying]

Because light is not known to pass through solid objects? This is how shadows are made. I'm unsure what your asking exactly

[amymadison182]

Okay... it truly depends on people see very very dark gray or very dark reddish-brown with eyes closed, eyelids are not 100% opaque, but they block light.

[moontokkym]

If you're looking directly into very bright light then you will still be able to see the light somewhat through your eyelids (like on a sunny day if you're in a car going under trees and close your eyes, you'll still be able to see the flashes of light as you go in and out of the shadow of the leaves).
Human skin is translucent, not fully opaque, that's why if you shine a torchlight through your ears the light seems to make your ears bright red.

I have light colour eyes and am very sensitive to light, I would probably die without my eyelids lol.

Also like others have said, eyelids aren't just for blocking out light, it's to protect against other objects, eyelashes are commonly thought to have evolved to catch sand and fur and the like before it gets in your eye. Also we live on a ball where there's another ball made of explosions in the sky that will burn our eyes out if we look at it for more than a second and that thing moves, so it's very easy to accidentally look at it. Being able to regulate how much light you get by squinting is basically nature's sunglasses.

[amymadison182]

Okay, but due to my skin being translucent, I don't see flashes of light going in and out of the shadow of the leaves as I go when I close my eyes, and I don't drive a car.