If something's unforgettable... how can you forget it?
Great ideas can come to you at any time, day or night. That's why I make it a point to write them down as soon as possible, ESPECIALLY when I think they're so obvious that I'll never forget them, because time after time, those are the ones I definitely forget.
Writing down your ideas not only insures you won't forget them, but it also frees your mind to relax and focus on other things, which makes you more productive as a whole.