Fatigue is generally defined as a feeling of lack of energy and motivation that can be physical, mental or both.

Fatigue is not the same as drowsiness, but the desire to sleep may accompany fatigue.
Apathy is a feeling of indifference that may accompany fatigue or exist independently.
In addition, individuals often describe fatigue using a variety of terms including weary, tired, exhausted, malaise, listless, lack of energy and feeling run down.

Fatigue is common. About 20% of Americans claim to have fatigue intense enough to interfere with living a normal life. A physical cause has been estimated to be responsible 20% to 60% of the time, while emotional or mental causes comprise the other 40% to 80% of cases of fatigue. Unfortunately, fatigue can also occur in normal individuals that experience intense physical or mental activity (or both).