Yeah, very cool isn't it? I take piano lessons, and my brain will play stuff for me in that state [edit: the half asleep state, not in piano class!] too... or sometimes make up stuff (!). And... it's often in the style that I've been listening to (so if I'm listening to alot of cool jazz stuff, that's what I'll hear.)

I have NO clue why it happens. I have a little bit of a guess through. You know how the brain is divided in half - the left brain is analytical and wordy, and the right brain is more creative and sensory based?

I think that what you're hearing is coming from your right brain. Because it's such a detailed and accurate sound... as if it were actual, real music (at least for me). Also, I've noticed if I try to think about it too hard (when it's new music for me, I start trying to analyze it better so I can write it down) - it immediately fades and disappears. This frustrates me so much... !

It's very cool though, and I've learned to just sort of enjoy while trying to not think too much, so as not to scare it away!

I'd be curious if you find any other info about it, because I haven't heard anyone really talk about it before... thanks!