NEWS YOU CAN USE: How Intense Study May Harm Our Workouts. “Tire your brain and your body may follow, a remarkable new study of mental fatigue finds. Strenuous mental exertion may lessen endurance and lead to shortened workouts, even if, in strict physiological terms, your body still has plenty of energy reserves.” This doesn’t surprise me. It’s one reason I started, years ago, working out in the morning instead of the afternoon.