Lixionary

This refers to that strange feeling when you lie down to go to sleep, and as you start to drift off your body is jolted for a second by what seems like an electric shock. In this moment, all of your muscles spasm so strongly that you almost fall out of bed, and you wake up straightaway.

This phenomenon is the result of the fact that when you begin to fall asleep, the frequency of your breathing rapidly falls, whilst your pulse slows down only very slightly and your muscles are relaxed. Amazingly, your brain interprets these developments as heralding death. So it tries to save you by giving you a jolt.

source: https://brightside.me