I had this sleeping disorder for a couple of months when I was a teenager, and I think it was because of my very stressful "first relationship". I would fall asleep almost anytime I would sit somewhere comfortable or even standing up in the shower.
When falling asleep like that, I had these short dreams about my boyfriend leaving me. At first, they were innocent: the two of us in a room, he turns around and walks out the door. But they became more violent, that he would be dragged away or killed before my eyes, that I would be beat up before his eyes, or that he would be the one killing me before he'd walk away.
I dared not speak about this to anyone, I felt really weird admitting it. I then trusted my best friend, and she advised to talk about it with my boyfriend. We had a nice conversation, where we dissolved all trust issues and jealousy between the two of us. I had the dream once more, when I fell asleep in his arms. I woke up in tears, and he said: "Don't worry, I'll be here for you". Never had that dream or sleeping problem ever since.
True story, for those sceptic people out there. :) What needs to be added, is that we broke up two months after that and he ended up being really immature about it. Teenagers, huh?