Workouts should last no longer than 60 minutes. After an hour, testosterone (a.k.a. the muscle-building hormone) declines as cortisol levels, which are associated with weight gain, rise. Workouts provide the best results when they are intense but short. (Sarah recommends hitting the gym no more than five times per week.)
Strength-training causes tears in your muscles, which repair between workouts and are what lead to their growth. If you don't allow adequate time for this recovery process, you can do more harm than good. Hold off on working a muscle group further until soreness has subsided.
Overtraining can wreak havoc on your hormones and actually lead to a weight-loss plateau or even weight-gain. Rest will help regulate these hormones.
The harder you train, the more fuel your body requires to keep up. Intense cardio, in particular, can lead to an increase in appetite and sabotage your weight-loss efforts.
Excessive strenuous exercise combined with a low body-fat percentage can lead to a drop in estrogen and missed periods. This can often be reversed by reducing the frequency and intensity of workouts. See your doctor if you miss your period for three months or more.
Having trouble falling asleep or feeling like you can't get enough sleep may be indications that you're overdoing it at the gym.
If you are exhausted throughout the day or find that your energy is dwindling when it comes to your workouts, your body is trying to tell you something: You're overdoing it, and it's time for a rest.