I've ever told a few people about this before, but I thought I should throw it out here. This phenomenon started as early as while I was in Primary school, but took a major break. I remember hearing interesting songs while entering Stage 1/2 of sleep, with the conscious still somewhat active to form memories, and capable of thoughts, while the subconscious had already begun its creative works. The earliest song I remember forming had something to do with SENTOSA and its acronym of So Expensive and Nothing To See Actually. It must be noted that such a song is 100% original at least in lyrics, if not in melody as well, since there's no such song out there at that time (or inaccessible to public at least), and probably isn't till this day.
It was very rare for that to happen though, and I thought nothing about it at that time. However, I joined MEP in secondary school, and gained more exposure to orchestral music. This led to a change of what songs I would form while I was half asleep. None other than orchestral naturally. It became more frequent towards Sec 4, and I was quite happy with it. It was like listening to an mp3 player while going to sleep. Except, most songs were original and unique. However, I retained little memories of those songs, since it was while I was GOING to sleep, and by the time I woke up, memories of my dreams had replaced those.
Considering I had started practicing lucid dreaming in Sec 2, dream recall and maintaining consciousness while asleep or half asleep became a skill I managed to learn. This allowed me more access to these melodies, and as I paid more attention to them, I have managed to muster the will to wake up and hum them into a recording device for preservation. Thus I've built up a collection of excerpts from my dreams. My humming never does them justice though, since what I hear are indeed full orchestras playing, and I can only hum the main melody line. Nevertheless, it helps with recollection later when I wake up.
Penning such pieces down is a real chore, and so far, I've only created one full orchestral piece, which wasn't even created while half asleep. It was while vacuuming the house and hearing the constant pedal G in the vacuum cleaner. I compose by inspiration, and that happens randomly, and at random times. However, my subconscious is a better composer, and usually I just need to seed it with the appropriate conditions and theme/motif melody to let it turn that into a full orchestral piece.
I don't have to be sleeping to experience music composition as such, simply spacing out entirely, like getting into stage 1 (alpha stage?) is sufficient for me to start producing melodies in my mind. They aren't as clear, beautiful, nor complex, like when I'm deeper in sleep, but at least they're easier to record, and I do have a few composed during train rides (undergrounds are the most.boring.ever.)
Just last night, due to me not being fully asleep (I was constantly thinking about an email, and when to check to reply), I "heard/composed" a total of 3 songs, of which now, I can't even remember how they sound like. However, I still have memories of my thoughts during the "playing" which is along the lines of "Wow, this is a really nice piece, I should totally get up and hum it down.... wait... it's getting nicer, oh wow! Lovely. Wait... how did that part start again? Oh shit, I've lost it. Nvm, I'll enjoy it while it lasts. I never get down to penning the pieces down even if I do record an excerpt anyway. And it takes too long to turn it into a full piece... Hey! The number of instruments here is awesome, I didn't know I could do that. Check out the horns (french horns). (And I really did listen intently to the horn parts) I'm going to hum along... (I did so in a sense)...."
The middle of the piece was really thick, with many layers of instruments which I could individually pick out. It was as if I was doing MEP listening all over again. I didn't compose the piece bit by bit, but rather, composed it as I was hearing it. I could actually direct the piece, like when I wanted something dramatic, it would turn dramatic (of course, evenly, like after 4 bars or something, so it didn't sound weird). It's fantastic. But this meant that I didn't know the notes or any details of the piece. I just knew what it sounded like as though I was listening to it the very first time (and I was anyway).
I don't actually know why this happens, but I really would love to get these pieces down, just that I don't have the time to do so. Each full orchestral piece would take me on the order of 12 hours, and it has to be done immediately upon waking up. The memory retention is extremely short, since the information is tremendous. It's like saying you need to create a movie out of your dream scenes, replicating every detail. I just lose most of the info when transiting from semi-conscious to full consciousness.
That's why I really hope, one day, there'll be a way to record sounds of thoughts, like your inner voice and stuff like that. That way, I could just record it down in entirety, and boy, what nice pieces would I have composed!
Oh yea, forgot to mention, because of this "ability", I can get pretty epic soundtracks during some dreams. Again, it's all lost when I wake up. Very unfortunate.