So tired..

Fatigue (also called exhaustion, languidness, and lassitude) is a weariness caused by exertion. It can 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.[1][2][3] It is ubiquitous in everyday life, but usually becomes particularly noticeable during heavy exercise. Mental fatigue, on the other hand, rather manifests in somnolence.

Mental fatigue
See also: Somnolence
In addition to physical, fatigue also includes mental fatigue, not necessarily including any muscle fatigue. Such a mental fatigue, in turn, can manifest itself both as somnolence (decreased wakefulness) or just as a general decrease of attention, not necessarily including sleepiness. In any case, this can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. For instance, a person who is sufficiently somnolent may experience microsleeps. However, objective cognitive testing should be done to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.
Causes
Fatigue is typically the result of working, mental stress, over stimulation and under stimulation, jet lag or active recreation, depression, and also boredom, disease and lack of sleep. It may also have chemical causes, such as poisoning or mineral or vitamin deficiencies. Massive blood loss, resulting in anemia, frequently results in fatigue.
The sense of fatigue is believed to originate in the reticular activating system of the lower brain. Musculoskeletal structures may have co-evolved with appropriate brain structures so that the complete unit functions together in a constructive and adaptive fashion.[4] The entire systems of muscles, joints, and proprioceptive and kinesthetic functions plus parts of the brain evolve and function together in a unitary way.[5]

http://en.wikipedia.org/wiki/Fatigue_%28medical%29