system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why do our bodies move involuntarily while we are sleeping?
gpt
Your brain is processing your memories and thoughts for the day. As well as creating your dreams. 

Your body wants to move along with those thoughts, but your brain also produces a powerful sedative that puts all your limbs to sleep, so that you don't move around and hurt yourself.

Sometimes you are able to break-free from that sedation and your limbs may jerk around a bit. Nothing is perfect.