That's usually, and normal, if you fall asleep quickly or if your body is just relaxing a little too quickly, your brain thinks your dying. Essentially your pulse and breathing slows so quickly just because you're *that* ready for sleep and *that* relaxed, that you're whole body just slowed down faster than your brain thinks is normal. So because your brain thinks you're body is dying is shocks you awake as a self preservation response

I get this too, so does my granddaughter. Her Dr explained it as the bodies way of resetting when you are extremely tired and your body is having a hard time relaxing. I don't know how true this is and I'm sure there is other explanations as well.

i have juvenile myoclonic epilepsy and my seizures are little jerks similar to that. i always had mine in the morning and around lunchtime. it’s normal to have these jerks when falling asleep, i wouldn’t worry about it unless it affects your day.