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 that go along with 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. Fatigue is a common symptom, and is usually not due to a serious disease.

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:

• An allergy that leads to hay fever or asthma
• Anemia (including iron deficiency anemia)
• Depression or grief
• Persistent pain
• Sleep disorders such as ongoing insomnia, obstructive sleep apnea, or narcolepsy
• Underactive or overactive thyroid gland
• Use of alcohol or drugs such as cocaine or narcotics, especially with regular use

Fatigue can also accompany the following illnesses:

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

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. Most people with CFS do not get much relief from rest.