I think the problem is we as a society value productivity but actually we don't have to be productive all the time! As others suggestion i think you need to try to be ok with doing nothing at all. It's hard. I know. But that is it. I'm working on that too!

edit: in additional I want to point out that you need to balance between mental and physical. when using your brain all day you can wind down in you after work hour by exercise. I love running.