I experience it from time to time, your in a state in between sleep and awake and can not move. Usually it's not such a bad thing unless your having a nightmare, then it can get nasty. Happened to me once. Usually it's just momentary paralysis. But the nightmare one seemed to last for hours and I was helpless. I was dreaming I was lifted off my bed and slammed against the wall. Over and over. I was also aware I was stuck and could not break free.