Chronic fatigue syndrome is a terribly debilitating illness that causes severe fatigue and other symptoms, such as:

• Impaired memory or concentration
• Sore throat
• Tender lymph nodes in the neck and under the arm
• Muscle pain
• Pain in multiple joints
• Headaches
• Unrefreshing sleep
• A worsening of these symptoms after exerting yourself

By definition, chronic fatigue syndrome must last for at least six months. Typically, it lasts for many years.

Getting rest helps most people with chronic fatigue syndrome feel somewhat better for a while, but it does not return a person to feeling healthy. Indeed, part of the definition of chronic fatigue syndrome requires that rest does not relieve the person's fatigue.

It is not known why rest — even prolonged rest — does not cure chronic fatigue syndrome. The most likely reason is that the disease process that is causing the fatigue is not much affected by resting. For example, people with chronic congestive heart failure often feel fatigued much or all of the time, and they too do not improve with prolonged rest. Presumably, that is because the fatigue from heart failure is caused by the fact that the heart muscle is not pumping as forcefully as the normal heart, and rest does not restore the strength of the weakened heart.

There is evidence that a very gradually-increasing exercise program can help many (but not all) people with chronic fatigue syndrome feel somewhat better. (Exercise does not cure chronic fatigue syndrome.)