Why do we shudder when fall asleep?
We tend to flinch not only when we fall asleep, but also when already sleep. The twitch we feel occurs when the two parts of the brain are out of sync with each other. The thinking part hasn't quite shut down yet, but the muscle control is already asleep.
As the thinking part cannot register any sensations that tell it where the body is at this moment and what it's doing, it's worried. The thinking part of the brain quickly "wakes up" the motor part by making a sudden movement to find any part of the body and give it some meaning.
-By Emmanuel Ametsikor