Sleep paralysis happens when you move in or out of REM sleep. During this stage, neurotransmitters like GABA and glycine basically render your muscles inoperative to make sure you don’t act out your dreams. So if you wake up before a REM cycle is over, your muscles would still be sleeping, and you wouldn’t be able to move, even if you’re fully awake. But what makes sleep paralysis so terrifying, and should you be worried?