It's called Sleep Paralysis. I have it,(and it can be very scary if you're not aware of what's going on) and All while growing up my mother use to tell me it was a demon. Everyone... when their in REM sleep are paralyzed. Sleep Paralysis is when you are half stuck in REM and half awake. You can not move one muscle or even talk. However "dreams" can take on a extremely odd reality. It is just a sleep disorder. Doesn't even happen all the time. Lots of time happens more to me when I'm "stressed". But now that I know what it is.. I try to will myself back to sleep! :)