system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do (in most cases) our eyes need to be closed for us to be able to sleep? And what happens to people who can sleep with their eyes open?
gpt
The physical reason animals sleep with their eyes closed is moisture. Just have a good old staring contest with someone and you will see after only a few minutes your eyes will start to dry out and the urge to blink really kicks in. Now imagine leaving them open for hours straight. As for sleeping with your eyes open, there are risks. If your eyes become too dry "dry spots" can form on the cornea, causing inflammation and over time could lead to a number of eye problems.