The Washington Post has run a great article about how body and brain fitness are one and the same.
http://www.washingtonpost.com/lifestyle/wellness/diet-and-exercise-resolutions-a-failure-maybe-youre-thinking-about-them-wrong/2015/01/20/8319b240-9b58-11e4-96cc-e858eba91ced_story.html
This is not theory or conjecture; it is proven over and over with scientific studies. Exercise improves overall energy levels and moods. A fit body also provides the brain with better nutrients, oxygen, and detoxification. A complete inability to exercise is extremely rare. So get creative and find regular activities that work out your body and your brain!