When I was at Princeton in the 1940s I could see what happened to those great minds at the Institute for Advanced Study, who had been specially selected for their tremendous brains and were now given this opportunity to sit in this lovely house by the woods there, with no classes to teach, with no obligations whatsoever. I believe that in a situation like this a kind of guilt or depression worms inside of you, and you begin to worry about not getting any ideas. And nothing happens. Still no ideas come. You don't have to think how to answer questions from the students.
Teaching is an interruption, and so it's the greatest pain in the neck in the world.
And then there are the longer period of time when not much is coming to you. If you're teaching a class, you can think about the elementary things that you know very well. These things are kind of fun and delightful. It doesn't do any harm to think them over again. If you do think of something new, you're rather pleased that you have a new way of looking at it.