Contrary to popular belief, it is possible to exercise too much.  This is called over-training.  It is a point in which a person is exercising so hard that their body does not recover.The body hits a point where performance begins to decrease (that is you start to lose strength and endurance).  The same workouts begin to get harder instead of easier and you lose that energized feeling after your workout.  Over-training is the result of either trying to do too much with not enough rest or trying to do too much too soon.  Symptoms of over-training include:

• Anger or depression
• Loss of appetite
• Bad sleeping patterns
• Prolonged body aches and soreness
• Muscle and strength loss
• Altered resting heart rate and blood pressure
• Menstrual irregularities in women
• Injury to a muscle or joint
• Flu or common cold that will not go away

The body needs rest in order to see improvements and gains from your workout.  Thus it is a good idea to have one or two days rest planned in your workout routine.  There is no true method to measure over-training such as blood test, etc..  If you notice the above symptoms, the best cure is to rest a day or two.  Even though it only affects a small percentage of those who exercise, one should talk to a fitness professional or health care provider on planning an exercise routine to prevent over-training and thus help you achieve your results.