I think there are other, very similar, threads here somewhere...
My best stuff comes to me in my dreams, but if I don't write it down RIGHT AWAY, after waking up, it is gone, Daddy, gone, in half an hour or less- all I am left with is the memory that I HAD a good song. Jotting down the lyrics is enough for me to recall the rest of it- melody, etc. stays with me then.
So, I have learned my lesson- when I literally "dream up a song," I get up and write it down, right away.
One song came to me FULLY FORMED in a dream- when I woke up, I was able to write down, with only pencil and paper at my bedside, all the lyrics, the melody, even chord diagrams, which I later researched via a "1,500 Guitar Chords" book. Even researching the chord names was easy- it was if I already knew them, just needed confirmation. There were some pretty strange chords in that song, too- many I had NEVER played before. I have no idea how that happened, but the song works quite well, and needed no editing or revision from the original notes.
Just last week, I dreamed up a song, but it faded completely, before my head was off the pillow. I decided to be philosophical about it- if it was that mecurial, it was not ready to be written, yet- if it comes back to me when it is ready, I'll write it, then.
I find songwriting to be about the most mystical thing I do. Lots about it I can not explain, and no longer try. On that note, let me leave you with this- Rumi expresses it much better than I:
Don’t Go Back To Sleep
the breeze at dawn has secrets to tell you.
don’t go back to sleep.
you must ask for what you really want.
don’t go back to sleep.
people are going back and forth between the
door sill where the two worlds touch.
the door is round and open.
don’t go back to sleep.
-rumi