Fatigue is a feeling of tiredness. It may be sudden or gradual in onset. It is a normal phenomenon if it follows prolonged physical or mental activity, and resolves completely with rest. However, it may be a symptom of a medical condition if it is prolonged, severe, progressive, or occurs without provocation.
Physical fatigue is the transient inability of muscles to maintain optimal physical performance, and is made more severe by intense physical exercise. Mental fatigue is a transient decrease in maximal cognitive performance resulting from prolonged periods of cognitive activity. Mental fatigue can manifest as somnolence, lethargy, or directed attention fatigue.
Fatigue and 'feelings of fatigue' are sometimes confused. Unlike weakness, fatigue usually can be alleviated by periods of rest.