Is it the one were you can't move your whole body?I happened to me alot of times, I know I'm sleeping but I can see my outside surrounding  ( bedroom ), I tried getting up, but it was just hard so as my feet, hands, head. It's like my spirit or me trying to get up but can't cause I'm still alive, it's pulling me back. Because that it had happened to me alot of times, my method is just be calm and try to bite my lip or move my whole body, it takes a while though.