Lethargy

n. drowsiness, listlessness, sluggishness, lack of energy; abnormal condition characterized by extreme sleepiness (Pathology)

The word fatigue is used in everyday living to describe a range of afflictions, varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles. It can be both physical and mental. Physical fatigue is the inability to continue functioning at the level of one's normal abilities. It is in everyday life, but usually becomes particularly noticeable during heavy exercise. Mental fatigue, on the other hand, rather manifests in somnolence.
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Lethargy /leth·ar·gy/ 1. a lowered level of consciousness, with drowsiness, listlessness, and apathy. 2. a condition of indifference. leth·ar·gy n. [more]Lethargy - Community and Resources

Noun
1. a state of comatose torpor (as found in sleeping sickness)
(synonym) lassitude, sluggishness
(hypernym) torpor, torpidity
(hyponym) hebetude
2. weakness characterized by a lack of vitality or energy
(synonym) inanition, lassitude
(hypernym) weakness
3. inactivity; showing an unusual lack of energy
(synonym) languor, sluggishness, phlegm
(hypernym) inactiveness, inactivity, inertia

(v. t.)
To lethargize.
  

(n.)
Morbid drowsiness; continued or profound sleep, from which a person can scarcely be awaked.
  

(n.)
A state of inaction or indifference.