The facts If you don't rest enough between hard cardio or strength workouts, you'll stop making progress and may even lose some of the fitness you've gained. You're also likely to burn out on exercise.

The fix To keep your muscles fresh and your motivation high, alternate shorter, tougher cardio workouts (for instance, 20 minutes) with longer, easier days (40-60 minutes). Don't go all-out more than twice a week. Keep in mind that the more intensely you train, the more time your body needs to recover. It's a good idea to do a couple of tough workouts and take 1 day completely off each week. On the strength-training front, take at least 1 day off between sessions that work the same muscle group.