i experienced this once, and have read about it too. waking(?) in the night and sensing the presence of some malevolent entity sitting close to or on my body, whilst at the same time being unable to move. happened to me during a period of extreme emotional strife, and i've always assumed that it was connected to that strife. any similar stories?
also ive read that it is one explanation for alien abduction experiences. i think that theory holds water