Why do our eyelids not block out all outside light when they are closed?
In: Biology
I would guess that it’s for light detection to regulate our sleep-wake cycle. It’s important to know when the Sun is coming up to avoid predators who hunt at night since they’re more visible during the day, to go out and about before the Sun is fully up and it’s hot, etc.
If you want a more literal answer, it’s because our eyelids aren’t thick enough to block out all light. It’s also important to remember that humans didn’t arise when indoor lighting was a thing.
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