When a brilliant new idea hits you, it's so clear, vivid, and powerful that you can't believe that you will ever forget it.
One day later, you might be scratching your head, saying, "I remember I had a great idea... but what was it?"
Memory is fickle, and this is especially true when you're dealing with different states of consciousness such as the expanded consciousness that accompanies inspiration. Get your idea written down, sketched, recorded, as soon as possible, in as much detail as you can manage.
It's still possible that you'll look at your notes a day later and say "I don't even remember why I thought this was such a great idea." But if the idea passes the 24-hour test, and still looks worthwhile the next day, you'll be glad you took the trouble to capture those details while they were fresh in your mind.