Weirdly, we now know that even the brain has light receptors. (The skull is very transparent when wet with CSF.) Night nurses know that a flashlight hitting a s

Weirdly, we now know that even the brain has light receptors. (The skull is very transparent when wet with CSF.) Night nurses know that a flashlight hitting a sleeping patients leg in darkness causes a contraction. Sleep masks help a lot - but they aren't a full solution.