I had these really bad about 11 years ago, can't pin point it down to one thing but it was very scary and it still happens from time to time but nothing like it did then. it's almost as if you can feel yourself slipping into the in-between stage of sleep and awake, most people aren't aware of this stage because it "just happens". I can tell you that when this started happening i started becoming more aware of myself, maybe you are too.