I used to get something like that. It was very similar to what kelly described although I don't recall ever going from that state back into sleep. I felt I had to get my mind and body reconnected and it was a great struggle. I would have to concentrate on moving a finger... then the hand....arm etc. When I was finally completely conscious and reconnected with my body I'd be exhausted and covered in sweat.
If you do a search you may find other posts on similar phenomena. Some people reported feeling a presence in the room with them. A supernatural presence.