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You detect light to know when to wake up or something is happening. Totally blind people also have the mechanism and why they know when it is morning (unless they are missing the organ). I am sure there are other reasons.
My guess, we evolved in hot areas so would often rest during the hottest part of the day. When you’re napping your brain can probably notice if a shadow blocks the light alerting you to a potential threat.
That’s totally an uneducated guess though.
Probably so you wake up when it’s light out
I’m now imagining falling asleep around a campfire, and there’s… a shadow
Funnily enough I know how this would feel! I gave myself a very bad black eye during a K-hole which burst a vein above my eye causing my eye lid to swell.
It felt like having a very hot and ‘sweaty’ eye ball covered by a winter duvet. It was so uncomfortably hot I actually got a mini fan to blow air into my eye while I held up the swelling.
Thankfully it resolved itself but thick eye lids are not good.
More of a story than I was expecting, coming to a thread about eyelids!
Thanks for passing on the knowledge
