Fatigue, also referred to as tiredness, exhaustion, lethargy, and listlessness, is a physical and/or mental state of being tired and weak. When somebody experiences physical fatigue, it impairs their ability to function at their normal levels of physical ability. Mental fatigue is characterized by feeling sleepy and being unable to concentrate properly.
Chronic fatigue syndrome (CFS) is an illness in which mental fatigue presents itself with often debilitating results, for periods of 6 months or more (3 months in children or adolescents. Fatigue is a common symptom in many illnesses, but CFS is comparatively rare.