The saying "No pain no gain" has its merits. It is no secret among athletes that you have to work hard in order to improve your performances and attaining your goals. Our body works like a fine tuned balance, on which everything has its designated place, timing and amount. Too much or too little of anything will disrupt that balance, and your body will eventually suffer. We all know that regular exercise is good for health, but too much exercise can also be harmful. Improvements from exercise only occur during the resting period after hard training, where your body tries to compensate the stress exerted on the cardiovascular and muscular systems by improving the efficiency of heart, muscles and energy production/utility, resulting in a higher level of performance. This process is called over-compensation, the body's way to keep itself in balance with demand.
Doing exercise without sufficient rest will not allow the body to have enough time to recover and regenerate, and consequently preventing the occurrence of over-compensation. This imbalance between excessive exercise and inadequate rest will eventually results in fatigue, decreased performance and "staleness". If not corrected, prolonged imbalance will stress the athletes to the point where resting is no longer adequate for recovery, this state is called overtraining. The symptoms of overtraining differ from daily fluctuations in performance that all athletes experience, it is characterized by a state of exhaustion that persists even after a period of recovery.
The most common symptom of overtraining is fatigue, but it does a lot more to your body than just making you feel tired. Overtraining can also alter your psychological state and immune system, increases the level of stress hormones in your body, decreases testosterone levels, increases muscle breakdown, and causes adrenal insufficiency. It may take from a few days up to months of resting to recover from overtraining depending on the severity of the symptoms. During this period many will experience exercise withdrawal and therefore should do moderate amount of exercise to manage the symptoms.
Exercise can be chemically addictive due to its effects on hormone levels in the brain. This may result in the fixation/dependence on exercise, which can lead to overtraining. A stressful training plan that does not incorporate enough recovery periods can also cause overtraining. As with everything in health, prevention is the best guarantee to a good life. Understand your body and be discreet with your training plans will do wonders to your health. Sometimes, having a good rest won't ruin your fitness, quite the contrary, it might just be the missing ingredient required to attain your next goal.