I'm a musician. I have this a lot because music is always stuck in my head. That's just what I think it is. Listening to the music that's stuck in your head but the thing about being half asleep is that things that might not make sense make perfect sense. So you may not actually be hearing the music down to the last detail but it feels like that and it feels more real. For example, my quintet is playing this piece that I have stuck in my head all of the time. When I was falling asleep, I heard the music and I thought to myself that the piece sounded exactly like mowing the lawn. And I was was really into that interpretation of the piece while I was falling asleep. I was planning on telling my group that and playing it while thinking of lawn mowers. And then I woke up and I was like .... what? It was an absolutely horrible interpretation but my half asleep self thought it was absolutely gorgeous and groundbreaking.