system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes the feeling of tripping and falling and the accompanying body movements when lying down usually around when you are trying to sleep?
gpt
I was told it's a old evolutionary throwback to when our ancestors used to live in trees. If the body felt like it was tipping over it jerked us awake so we would grab hold of the branch.

As we now sleep in beds it sometimes gets confused and activates.

The trait never dissappeared with evolution because there is no benifit to not having it, so it made no difference to reproduction rates if anyone was born without it.