Fatigue describes a state of tiredness or exhaustion. In general usage, fatigue often follows prolonged physical or mental activity. When fatigue occurs independently of physical or mental exertion, or does not resolve after rest or sleep, it may have other causes, such as a medical condition.
Fatigue (in a medical context) often has no known cause, and is recognised as being complex in nature. Fatigue may be associated with conditions of chronic pain such as fibromyalgia, and with many other conditions, including autoimmune diseases. It can also be associated with mental disorders such as depression.
Fatigue (in the general usage sense of normal tiredness) can include both physical and mental fatigue. Physical fatigue results from muscle fatigue brought about by intense physical activity. Mental fatigue results from prolonged periods of cognitive activity which impairs cognitive ability. Mental fatigue can manifest as sleepiness, lethargy, or directed attention fatigue. Mental fatigue can also impair physical performance.