When athletes don't get enough rest between workouts, they can become fatigued and listless. They may have trouble sleeping. And they can become more susceptible to colds and other illnesses. Female athletes may find their periods slow or stop during heavy training.

Even if you're not an athlete, but you exercise for good health, resting between workouts time is important. Depending upon how hard you exercise, you need to rest one to two days per week. This approach is supported by surveys that show injuries increase when people perform high intensity exercise more than five days per week.

If you lift weights, you need even more rest -- at least 48 hours between workouts. Don't lift weights more than three times per week, experts say.

And always listen to your body. If you exercise most days of the week, and feel run-down, have trouble sleeping or feel that your workouts are tougher than usual, cut back on your schedule. (Cut back how often you exercise and how hard you exercise). You can go two to three weeks without working out before your fitness level starts to slip.

If you continue to experience fatigue despite getting some rest, talk to your doctor.