Basically, there are three types of fatigue: Physical, pathological (由疾病引起的), and psychological. As you might suspect, each differs significantly from the others.
    When you exercise your body you produce waste products. Muscles, for example, discard lactic acid (乳酸) into the blood; cells dump in carbon dioxide. When these wastes reach a certain level in the blood, the brain is notified and your activity level drops. Excess wastes in the muscles may produce soreness. If the blood of a physically fatigued animal is injected into a rested animal,it will produce fatigue. The solution to this type of fatigue is simple - rest. That should revive you;if it doesn't, another cause should be sought.
    Have you ever become involved in so many activities that you had to be in two places at once? This is what happens when your body has a disease. The cells are overtaxed and cannot keep up with both fighting the disease and keeping you active. The result is fatigue. Some communicable diseases like the flu and colds are notorious for draining your energy. Other non-communicable diseases, like anemia (贫血), drain you because you are lacking an important body ingredient. Being overweight can cause pathological fatigue. It should be obvious that this type of fatigue is not going to go away without treatment. In a way, pathological fatigue is a lifesaver. It lets you know something is wrong and that you need rest. Even a poor diet can produce pathological fatigue. Frequently, people who go on crash diets develop pathological fatigue, and if the diet is not improved, they may do physical harm to their bodies.
    Here is the most common type of fatigue. Almost everybody experiences it now and then. Often, the cause is an emotional war you are waging with yourself or those around you. Some of these familiar factors can bring on psychological fatigue: Worries, stress, lack of exercise, boredom,depression. If you know someone with psychological fatigue, would you advise him to rest? No way! That might be fine for our other types of fatigue, but for this one, it's deadly. If you are ever going to be able to cope with stress, depression, or worry, you need oxygen in your cells and a more optimistic attitude. Get out of the chair and do something! Believe it or not, many people throw themselves into physical labor like cleaning or carpentry (木工活) to "defatigue" themselves.If you find yourself in a particularly stressful situation that you can't physically escape, escape mentally. When fatigue continues, maybe you need to get to the root of the problem.