Signs and symptoms may include:

• Fatigue
• Loss of memory or concentration
• Sore throat
• Enlarged lymph nodes in your neck or armpits
• Unexplained muscle or joint pain
• Headaches
• Unrefreshing sleep
• Extreme exhaustion lasting more than 24 hours after physical or mental exercise

People who have chronic fatigue syndrome appear to be hypersensitive to even normal amounts of exercise and activity.

Why this occurs in some people and not others is still unknown. Some people may be born with a predisposition for the disorder, which is then triggered by a combination of factors. Potential triggers include:

• Viral infections. Because some people develop chronic fatigue syndrome after having a viral infection, researchers question whether some viruses might trigger the disorder. Suspicious viruses include Epstein-Barr virus, human herpes virus 6 and mouse leukemia viruses. No conclusive link has yet been found.
• Immune system problems. The immune systems of people who have chronic fatigue syndrome appear to be impaired slightly, but it's unclear if this impairment is enough to actually cause the disorder
• Hormonal imbalances. People who have chronic fatigue syndrome also sometimes experience abnormal blood levels of hormones produced in the hypothalamus, pituitary glands or adrenal glands. But the significance of these abnormalities is still unknown.

Factors that may increase your risk of chronic fatigue syndrome include:

• Age. Chronic fatigue syndrome can occur at any age, but it most commonly affects people in their 40s and 50s.
• Sex. Women are diagnosed with chronic fatigue syndrome much more often than men, but it may be that women are simply more likely to report their symptoms to a doctor.
• Stress. Difficulty managing stress may contribute to the development of chronic fatigue syndrome.

Possible complications of chronic fatigue syndrome include:

• Depression
• Social isolation
• Lifestyle restrictions
• Increased work absences