Lassitude

Fatigue describes a state of tiredness (which is not sleepiness), exhaustion or loss of energy.

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 the medical sense) is associated with a wide variety of conditions including autoimmune disease, organ failure, chronic pain conditions, mood disorders, heart disease, infectious diseases, and post-infectious-disease states. However fatigue is complex and in some 50% of cases its cause is unknown after one year.

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, and can also impair physical performance.

Source: Wikipedia