I've also experienced it few times. For the first time I was so freaked out. I didn't hallucinate any demons and boogeyman but I remember the horror of paralysis when you think/feel you're awake. And the feeling of uncontrollable spinning around, awful. I've experienced it few more times later on but I didn't panic as it was familiar to me and the experience wasn't that bad (but far from pleasant). I can relate it with tiredness, like when you're so tired but you can't sleep. That's when it kicked me. I don't know if it's related to psychological trauma, possible. But maybe there is some sort of electrolyte imbalance that could also be the cause of it. To me it smells more like a neurological story, rather than psychological as it happened to me when I was physically exhausted. Could be both tho. Great post anyway! :)