There's emotional stress, and then there's physical stress. If you do a lot of high-intensity workouts, you may notice that you’re often injured or ill. When you exercise, your central nervous system sends signals to your brain and muscles to let them know what’s going on and what's needed. Frequent exercise is good for this system, but exercising too frequently trains it to always expect the next round of exercise; consequently you never really relax. Your body needs recovery time (so maybe now's the time for that box set).