I’ve done this since I was 7 or 8. Been to numerous doctors and had sleep studies. Been told this is caused when my body goes from stage 1 sleep cycle straight to stage 3. If you skip the stage 2 sleep cycle it throws your body into a somewhat shock and the brain misfires sending the nerves and muscles into a electric storm. There are 4 stages to sleep and your body has to go through them in number order to be happy. I don’t know tho, maybe the Old Hag. I know it sucks. I also jerk my head quit violently. I took a nap today after work and I guess my body was really tired because I did it again. My neck hurts pretty bad now cause I guess I was trying to wake myself up from the nightmare too hard.