That happened to me once in my life, but waking up from a dreamless sleep with a new idea is fairly normal for me (to the point where I consider sleeping a part of my problem-solving strategy).
When my subconscious is working on a problem, I have this pleasant lazy feeling similar to the feeling I get after finishing productive physical labor. It makes me want to relax and daydream. Do you get something like that?
One time when I was in college, I was in chemistry class. It was a class I had a lot of trouble with but this time I was actually able to follow along with the professor and take good notes. Then I woke up, I was in chemistry class, but I had slept through half the lecture.