Oddly enough, I had something very similar last night; I was convinced that there was a vampire in the room with me, I never saw him but I dare not look to see if it was actually there for about 15 minutes. Then I decided I was too tired and didn't care if there was a vampire there or not and told it to get lost, then turned the other way to get back to sleep.

Last time I had anything similar was about ten years ago, this time I did see something; it was a really tall dark figure that I assumed was a ghost drifting into my room, I laughed at it and told that one to get lost too, and it just went away. I think it was this earlier experience that gave me the idea to tell last night's visitor to get lost. I'm not sure what I would have done had the ghost decided to stay.

As for scientific terms for your experience, I suppose you could call it an advanced form of Lucid Dreaming?