[paul.fr]

Why when you close your eyes, does light get through? I know the eye lid is thin but does letting some light through serve a purpose?

[RD]

If eyelids were totally opaque then the pupil would adapt to total darkness during a blink and dilate to its maximum size, then when the eyelid was opened vision would momentarily be compromised by overexposure, (pupils do not change size instantaneously).
So totally opaque eyelids are not a good idea.

[another_someone]

In any event, I would imagine that even when you are asleep, you want to be aware of night and day, otherwise (in the days before the alarm clock), how would you know when it was time to get up?