Repeated sessions of deep concentration in a particular subject or problem over a prolonged period clearly creates neural growth. I've tackled several extremely complex projects in my life, and in each case I found that I became increasingly proficient at working on them as time progressed. I started to realize epiphanies typically aren't that you suddenly discovered something, but are that your cognitive ability ultimately progressed to the point where you can fully understand something. By the way, in each of these periods I was regularly napping because of the mental fatigue. I have a sense that a lot of the neural growth happened during those naps.