system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: REM stands for Rapid Eye Movement. What causes/is the purpose of our eyes to still move during sleep when the rest of our body is nearly inhibited and our eyes are closed the whole time?
gpt
Short answer: no one knows. 

Longer answer:
First of all, REM is a bit of a misnomer. The eye movements during REM are actually slower than while you're awake - they're just much faster than in non-REM sleep.

There are two main ideas as to what's going on.
The things you see in dreams are obviously not based on images from your eyes. These images are built in the visual centres of your brain though. During normal vision, there's constant feedback to the eyes from the visual centres to help with following something you're interested in, focus, and so on. Even though the eyes aren't sending much information to the brain, the brain doesn't stop sending feedback to the eyes based on the images it's made up. 

This might be the cause of REM. It makes sense, but it's not perfect. People who were born blind still move their eyes (if they physically can) during REM, in a way that doesn't match their eye movements when awake. Keep in mind that your eye nerves are directly plugged into the visual centres of your brain. Most of the rest of your body goes via the spinal chord and brain stem.

There's also the idea that the REM phase of sleep might be for actively processing memory. In this view, your brain is playing back memories kind of like a tape, and your eyes are playing back the movements that went with those memories. This could be a side-effect, or it could actually help in processing.