That's the way it comes to me too for some reason.
I don't know why but I can be half asleep and I'll get this great idea and I always think... "aw... I'll remember it in the morning". Next day I don't have a clue what the song was.

Now I do the same thing... keep a digital tape recorder around and even if I just hum the notes into it or whatever, at least I'll remember what I was thinking the night before.

(an iPhone is awesome btw: perfect for recording quick notes or even 4 track recordings... hell, you can even take a pic of yourself and track where you were with the GPS and use a guitar application to figure out the leads
Then you can record your basic ideas in Cubase, convert them to MP3's and store them on the phone and use a pair of BlueTooth Sunglasses to listen to your own song while you're in the car. :)