"Exercise to discharge your unused daily energy Stephen Dubner frames exercise not as discipline but as maintenance. He explained that the body's energy is "like a battery that requires regular use" and that "when you don't exercise, that unused energy overflows and causes weird behavior, overreacting to things, and restlessness." Physical activity is not optional maintenance — it is how the body releases the charge it built up for demands that never came. Use it or it uses you. Source: Stephen Dubner, Freakonomics Radio, How to Be More Productive"