Fatigue is a subjective feeling of exhaustion or tiredness which is distinct from weakness, and may has a gradual onset. Unlike weakness, fatigue may be alleviated by periods of rest. Fatigue may have as physical, as mental causes. Physical fatigue is can occur after intense physical activity. Mental fatigue is a transient decrease in maximal cognitive performance - a result from prolonged period of mental activity. It may manifest as lethargy, somnolence or as directed attention fatigue.