Your brain needs exercise, too!

Physical

The brain is organ tissue, not a muscle like the muscles in your arms and legs, or even your heart. However, physical exercise is good for the brain. Physical exercise that strengthens your leg, arm and/or heart muscles also helps strengthen your brain! Exercise increases the flow of oxygen and other nutrients to the brain. (Kurland and Lupoff, 1999, p. xx)

Mental - Use it or Lose it!

The phrase, "Use it or lose it!" does have a degree of truth in it. Using your brain for problem solving and developing your memory actually does help you think and memorize better. The neurons in your brain will branch out and make new connections. (Kurland and Lupoff, 1999, p. 103).

Meanwhile
Some people use time while exercising for rehearsal or repetition of facts that they are trying to memorize. This is a good use of time!