system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does our body "twitch" as we are falling asleep sometimes?

Is it like a misfire or do we feel like our body is suddenly falling and overreact?
gpt
It's called a hypnic jerk.  

Why we do it as we fall asleep is a matter of speculation.  The current idea is that it's a holdover from primate days and it helped to ensure that we were secure where we were sleeping.  Basically is was your brain's way to tricking your body into thinking it was falling to make sure it reacted appropriately (and you didn't lose your balance and fall) before you went to sleep.