Fatigue (also called exhaustion, lethargy, languidness, languor, lassitude, and listlessness) is a state of awareness. 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. It is ubiquitous in everyday life, but usually becomes particularly noticeable during heavy exercise. Mental fatigue, on the other hand, rather manifests in somnolence (sleepiness).

Fatigue is typically the result of working, mental stress, overstimulation and understimulation, 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 frequently results in fatigue. Fatigue is different from drowsiness, where a patient feels that sleep is required. Fatigue is a normal response to physical exertion or stress, but can also be a sign of a physical disorder.

Fatigue is considered a symptom, as opposed to a medical sign, because it is reported by the patient instead of being observed by others. Fatigue and ‘feelings of fatigue’ are often confused.

Physical fatigue

Physical fatigue or muscle weakness (or "lack of strength") is a direct term for the inability to exert force with one's muscles to the degree that would be expected given the individual's general physical fitness.

Mental fatigue

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. It may also be described as more or less decreased level of consciousness. In any case, this can be dangerous when performing tasks that require constant concentration, such as driving a vehicle.

Temporary fatigue is likely to be a minor illness like the common cold as one part of the sickness behavior response that happens when the immune system fights an infection. Chronic fatigue, on the other hand, meaning of six months or more duration, is a symptom of a large number of different diseases or conditions.

Some major categories of diseases that feature fatigue include:

• Autoimmune diseases such as celiac disease, multiple sclerosis, and spondyloarthropathy
• Blood disorders such as anemia and hemochromatosis
• Cancer
• Chronic fatigue syndrome (CFS)
• Depression and other mental disorders that feature depressed mood
• Eating disorders, which can produce fatigue due to inadequate nutrition
• Endocrine disease like diabetes mellitus and hypothyroidism
• Fibromyalgia
• Heart disease
• Infectious diseases such as infectious mononucleosis and influenza
• Leukemia or lymphoma
• Neurological disorders such as Parkinson's disease and post-concussion syndrome
• Physical trauma and other pain-causing conditions, such as arthritis
• Pregnancy
• Sleep deprivation or sleep disorders
• uremia
• hepatic failure

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