Fatigue is a feeling of excessive tiredness or lethargy, with a desire to rest, perhaps to sleep. Often it causes yawning.
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 or deficiencies.
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.
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