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.
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 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.
A test of strength is often used during a diagnosis of a muscular disorder before the etiology can be identified. Such etiology depends on the type of muscle weakness, which can be true or perceived as well as central or peripheral. True weakness is substantial, while perceived rather is a sensation of having to put more effort to do the same task. On the other hand, central muscle weakness is an overall exhaustion of the whole body, while peripheral weakness is an exhaustion of individual muscles.
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. 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.
The majority of people who have fatigue do not have an underlying cause discovered after a year with the condition. In those who do have a possible diagnosis musculoskeletal (19.4%) and psychological problems (16.5%) are the most common. Definitive physical conditions were only found in 8.2%.
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.
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. The entire systems of muscles, joints, and proprioceptive and kinesthetic functions plus parts of the brain evolve and function together in a unitary way.
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
- 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
- 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
- Sleep deprivation or sleep disorders
- hepatic failure
- Certain medications, e.g. lithium salts, ciprofloxacin
- Beta blocker medication causes fatigue, especially after exertion, inducing exercise intolerance.
- Many cancer treatments cause fatigue, particularly chemotherapy and radiotherapy