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. Fatigue and 'feelings of fatigue' are sometimes confused. Unlike weakness, fatigue can usually be alleviated by periods of rest.