Trying to fall asleep on a recent flight made me wonder this. If eyelids were opaque, it would make sleeping a whole lot easier.
Is there was some reproductive advantage to opaque eyelids they would be…
The reason they are translucent is that they are so thin, possibly, and not directly selected for or against translucency per se? I would like to know if people with very dark skin see much less light through their eyelids, too. A weird but interesting question. Maybe it is not a good trait, evolutionarily, to be unable to detect that it is getting light in the morning when you are asleep. Maybe when our ancestors were resting in the day, it was helpful to detect the shadow of a potential threat as it passed over their closed eyes.
Well, they’re really just skin, right? And skin is translucent.
If I hold a bright enough flashlight to my hand, I can see light through my hand. (Tinted red.)
Imagine if your eyelids were a couple inches or more thick.
Perhaps so that if you were sleeping during the day (when other predators might be out) you have at least some chance of noticing them approaching if their shadows crossed your face.
Not necessarily, not in the dark, anyway, and humans, as diurnal animals, are really adapted to sleeping in the dark. Trying to sleep in the light is largely because we have artificial lighting technology, and not only sometimes leave it on during “natural” sleep time (as in a plane on the “red-eye”) but it also enables us to indulge the bad (but fun) habit of staying up late and sleeping during the day. I think our eyelids evolved well before artificial lighting (even fiery torches) were invented, however.
Also, I find myself that it is not all that difficult to sleep in moderately bright light with my eyes closed but not covered. It is largely just a matter of getting used to it. I have heard that some people, and animals, are even quite capable of sleeping with their eyes open also. You may be bothered by the light through your eyelids, because you are not used to sleeping that way, but not everyone will be.
Just a thought - eyelids protect your eyes, not just block light. Lightweight eyelids would be able to close faster when the rock heads your way.
maybe allows you to wake up at dawn?