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Fatigue is a state, following a period of mental or bodily activity, characterized by a lessened capacity for work and reduced efficiency of accomplishment, usually accompanied by a feeling of weariness, sleepiness, or irritability. It may also follow when, from any cause, energy expenditure outstrips restorative processes and may be confined to a single organ.
It is typically the result of working, mental stress, jet lag or active recreation, but also from boredom or disease or simply lack of sleep. It may also have chemical causes, such as poisoning or mineral or vitamin deficiencies.
When chronic (meaning of six months or more duration) it is a symptom of nearly 30 different diseases. Post exertional fatigue, also known as exercise intolerance, is however far more rare, and is primarily found in organic brain diseases, mitochondrial disease, and neuromuscular disease.
Fatigue can also be quite dangerous when performing certain duties that require constant concentration, such as driving a vehicle. When someone is sufficiently fatigued, they may experience microsleeps that can cause them to lose concentration, however objective cognitive testing should be done to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.
The sense of fatigue is believed to originate in the reticular activating system of the lower brain.
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