Fatigue is a feeling of weariness, tiredness, or lack of energy.

Fatigue is different from drowsiness. In general, drowsiness is feeling the need to sleep, while fatigue is a lack of energy and motivation. Drowsiness and apathy (a feeling of indifference or not caring about what happens) can be symptoms of fatigue.

Fatigue can be a normal and important response to physical exertion, emotional stress, boredom, or lack of sleep. However, it can also be a nonspecific sign of a more serious psychological or physical disorder. When fatigue is not relieved by enough sleep, good nutrition, or a low-stress environment, it should be evaluated by your doctor. Because fatigue is a common complaint, sometimes a potentially serious cause may be overlooked.

The pattern of fatigue may help your doctor determine its underlying cause. For example, if you wake up in the morning rested but rapidly develop fatigue with activity, you may have an ongoing physical condition like an underactive thyroid. On the other hand, if you wake up with a low level of energy and have fatigue that lasts throughout the day, you may be depressed.

There are many possible physical and psychological causes of fatigue. Some of the more common are:

• Anemia (including iron deficiency anemia)
• Sleep disorders like ongoing insomnia, obstructive sleep apnea, or narcolepsy
• Ongoing pain
• An allergy that leads to hay fever or asthma
• An underactive thyroid (hypothyroidism)
• Use of alcohol or illegal drugs like cocaine, especially with regular use
• Depression or grief

Fatigue can also accompany the following illnesses:

• Infection, especially one that takes a long time to recover from or treat, like bacterial endocarditis (infection of the heart muscle or valves), parasitic infections, AIDS, tuberculosis, and mononucleosis
• Congestive heart failure
• Diabetes
• Chronic liver or kidney disease
• Addison's disease
• Anorexia or other eating disorders
• Arthritis, including juvenile rheumatoid arthritis, which affects children and teens
• Autoimmune diseases such as lupus
• Malnutrition
• Cancer

Certain medications may also cause drowsiness or fatigue, including antihistamines for allergies, blood pressure medicines, sleeping pills, steroids, and diuretics.

Chronic fatigue syndrome (CFS) is a condition that starts with flu-like symptoms and lasts for 6 months or more. All other possible causes of fatigue are eliminated before this diagnosis is made. Little relieves CFS, including rest.

Ressel GW. National Institutes of Health. NIH releases statement on managing pain, depression, and fatigue in cancer. Am Fam Physician. 2003; 67(2): 423-424.

Penninx BW. Anemia and decline in physical performance among older persons. Am J Med. 2003; 115(2): 104-110.

Williams RH, Larsen PR, eds. Williams Textbook of Endocrinology. Amsterdam, Netherlands: Elsevier; 2003.