Occupations that require an individual to work long hours or stay up overnight can lead to fatigue.
Fatigue (also called exhaustion, tiredness, lethargy, languidness, languor, lassitude, and listlessness) is a subjective feeling of tiredness which is distinct from weakness, and has a gradual onset. Unlike weakness, fatigue can be alleviated by periods of rest. Fatigue can have physical or mental causes. Physical fatigue is the transient inability of a muscle to maintain optimal physical performance, and is made more severe by intense physical exercise. Mental fatigue is a transient decrease in maximal cognitive performance resulting from prolonged periods of cognitive activity. It can manifest as somnolence, lethargy, or directed attention fatigue.
Medically, fatigue is a non-specific symptom, which means that it has many possible causes. Fatigue is considered a symptom, rather than a sign because it is a subjective feeling reported by the patient, rather than an objective one that can be observed by others. Fatigue and ‘feelings of fatigue’ are often confused.
Physical fatigue 
Physical fatigue, or muscle fatigue, is the temporary physical inability of a muscle to perform optimally. The onset of muscle fatigue during physical activity is gradual, and depends upon an individual's level of physical fitness, and also upon other factors, such as sleep deprivation and overall health. It can be reversed by rest. Physical fatigue can be caused by a lack of energy in the muscle, by a decrease of the efficiency of the neuromuscular junction or by a reduction of the drive originating from the central nervous system. The central component of fatigue is triggered by an increase of the level of serotonin in the central nervous system. During motor activity, serotonin released in synapses that contact motoneurons promotes muscle contraction. During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extrasynaptic receptors located on the axon initial segment of motoneurons with the result that nerve impulse initiation and thereby muscle contraction are inhibited.
Muscle strength testing can be used to determine the presence of a neuromuscular disease, but cannot determine its etiology. Additional testing, such as electromyography, can provide diagnostic information, but information gained from muscle strength testing alone is not enough to diagnose most neuromuscular disorders.
Mental fatigue 
Mental fatigue is a temporary inability to maintain optimal cognitive performance. The onset of mental fatigue during any cognitive activity is gradual, and depends upon an individual's cognitive ability, and also upon other factors, such as sleep deprivation and overall health. Mental fatigue has also been shown to decrease physical performance. It can manifest as somnolence, lethargy, or directed attention fatigue. Decreased attention is known as ego depletion and occurs when the limited 'self-regulatory capacity' is depleted. It may also be described as a more or less decreased level of consciousness. In any case, this can be dangerous when performing tasks that require constant concentration, such as operating large vehicles. For instance, a person who is sufficiently somnolent may experience microsleep. However, objective cognitive testing can be used to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.
Fatigue vs. sleepiness 
Fatigue is generally considered a more long-term condition than sleepiness (somnolence). Although sleepiness can be a symptom of medical issues, it usually results from lack of restful sleep, or a lack of stimulation . Chronic fatigue, on the other hand, is a symptom of a greater medical problem in most cases. It manifests in mental or physical weariness and inability to complete tasks at normal performance. Both are often used interchangeably and even categorized under the description of 'being tired.'
Fatigue is a normal 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.
Chronic fatigue 
Prolonged fatigue is a self-reported, persistent (constant) fatigue lasting at least one month. Chronic fatigue is a self-reported fatigue lasting at least six consecutive months. Chronic fatigue may be either persistent or relapsing. Chronic fatigue is a symptom of many diseases and conditions. Some major categories of diseases that feature fatigue include:
- Autoimmune diseases such as celiac disease, multiple sclerosis, myasthenia gravis, and spondyloarthropathy
- Blood disorders such as anemia and hemochromatosis
- Cancer, in which case it is called cancer fatigue
- Chronic fatigue syndrome (CFS)
- Drug abuse including alcohol abuse
- Depression and other mental disorders that feature depressed mood
- Eating disorders, which can produce fatigue due to inadequate nutrition
- Endocrine diseases like diabetes mellitus and hypothyroidism
- Gulf War Syndrome
- Heart disease
- Irritable Bowel Syndrome
- Inborn errors of metabolism such as Fructose malabsorption.
- Liver failure
- Infectious diseases such as infectious mononucleosis.
- Leukemia or lymphoma
- Lyme disease
- Neurological disorders such as narcolepsy, Parkinson's disease and post-concussion syndrome
- Physical trauma and other pain-causing conditions, such as arthritis
- Sleep deprivation or sleep disorders
- Uremia, which is caused by kidney disease
Fatigue may also be a side effect of certain medications, e.g. lithium salts, ciprofloxacin; beta blockers, which can induce exercise intolerance; and many cancer treatments, particularly chemotherapy and radiotherapy.
One study concluded about 50% of people who have fatigue receive a diagnosis that could explain the fatigue after a year with the condition. In those people who 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%.
If a person with fatigue decides to seek medical advice, the overall goal is to identify and rule out any treatable conditions. This is done by considering the person's medical history, any other symptoms that are present, and evaluating of the qualities of the fatigue itself. The affected person may be able to identify patterns to the fatigue, such as being more tired at certain times of day, whether fatigue increases throughout the day, and whether fatigue is reduced after taking a nap.
Because disrupted sleep is a significant contributor to fatigue, a diagnostic evaluation considers the quality of sleep, the emotional state of the person, sleep pattern, and stress level. The amount of sleep, the hours that are set aside for sleep, and the number of times that a person awakens during the night are important. A sleep study may be ordered to rule out a sleep disorder.
Depression and other psychological conditions can produce fatigue, so people who report fatigue are routinely screened for these conditions, along with drug abuse, poor diet, and lack of physical exercise, which paradoxically increases fatigue.
Basic medical tests may be performed to rule out common causes of fatigue. These include blood tests to check for infection or anaemia, a urinalysis to look for signs of liver disease or diabetes mellitus, and other tests to check for kidney and liver function, such as a comprehensive metabolic panel. Other tests may be chosen depending on the patient's social history, such as an HIV test or pregnancy test.
See also 
- Cancer-related fatigue
- Combat stress reaction
- Fatigue (safety)
- Heat illness
- Sleep-deprived driving
- Gaucher's Disease
- Gandevia SC. "Some central and peripheral factors affecting human motoneuronal output in neuromuscular fatigue". Sports medicine (Auckland, N.Z.) 13 (2): 93–8 year=1992. doi:10.2165/00007256-199213020-00004. PMID 1561512.
- Hagberg M (1981). "Muscular endurance and surface electromyogram in isometric and dynamic exercise". Journal of Applied Physiology 51 (1): 1–7. PMID 7263402.
- Hawley JA, Reilly T (1997). "Fatigue revisited". Journal of sports sciences 15 (3): 245–6. doi:10.1080/026404197367245. PMID 9232549.
- Marcora, Samuele (January 2009). "Mental fatigue impairs physical performance in humans". Journal of Applied Physiology 106 (3): 857–864. doi:10.1152/japplphysiol.91324.2008.
- Berrios GE (1990). "Feelings of fatigue and psychopathology: a conceptual history". Compr Psychiatry 31 (2): 140–51. doi:10.1016/0010-440X(90)90018-N. PMID 2178863.
- "Weakness and fatigue". Healthwise Inc. Retrieved 2 January 2013.
- Gandevia SC (2001). "Spinal and supraspinal factors in human muscle fatigue". Physiol. Rev. 81 (4): 1725–89. PMID 11581501.
- Perrier JF, Delgado-Lezama R (2005). "Synaptic release of serotonin induced by stimulation of the raphe nucleus promotes plateau potentials in spinal motoneurons of the adult turtle". J Neurosci. 25 (35): 7993–9. PMID 16135756.
- Cotel F, Exley R, Cragg SJ, Perrier JF (2013). "Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation". Proc Natl Acad Sci U S A 110 (12): 4774–9. PMID 23487756.
- Enoka, R. M.; Duchateau, J. (11 October 2007). "Muscle fatigue: what, why and how it influences muscle function". The Journal of Physiology 586 (1): 11–23. doi:10.1113/jphysiol.2007.139477.
- Edelman, Gerald Maurice (1989). The remembered present: a biological theory of consciousness. New York: Basic Books. ISBN 0-465-06910-X.
- Kelso, J. A. Scott (1995). Dynamic patterns: the self-organization of brain and behavior. Cambridge, Mass: MIT Press. ISBN 0-262-61131-7.
- Baumeister, R. F. (2002). Ego Depletion and Self-Control Failure: An Energy Model of the Self’s Executive Function. Self and Identity, 1, 129-136.
- AJ Giannini Fatigue,Chronic. In RB Taylor Difficult Diagnosis--2. Philadelphia, WB Saunders Co.,1992,pg 156. ISBN 0-7216-3481-8
- Shen, Jianhua; Joseph Barbera, Colin M. Shapiro (February 2006). "Distinguishing sleepiness and fatigue: focus on definition and measurement". Sleep Medicine Reviews 10 (1): 63–76. doi:10.1016/j.smrv.2005.05.004. Retrieved 22 February 2012.
- Hoddes, E; V. Zarcone, H. Smythe, R. Phillips, W. C. Dement (30). "Quantification of Sleepiness: A New Approach". Psychophysiology 10 (4): 431–436. doi:10.1111/j.1469-8986.1973.tb00801.x. Retrieved 23 February 2012.
- Wessely, S.; M Hotopf, M Sharpe (9). "Chronic fatigue and its syndromes". BMJ 318: 416. Retrieved 23 February 2012.
- Fukuda K, Straus S, Hickie I, Sharpe M, Dobbins J, Komaroff A (15 December 1994). "The chronic fatigue syndrome: a comprehensive approach to its definition and study. International Chronic Fatigue Syndrome Study Group.". Ann Intern Med 121 (12): 953–9. doi:10.1059/0003-4819-121-12-199412150-00009. PMID 7978722.
- Avellaneda Fernández, A.; Pérez Martín, A.; Izquierdo Martínez, M.; Arruti Bustillo, M.; Barbado Hernández, FJ.; de la Cruz Labrado, J.; Díaz-Delgado Peñas, R.; Gutiérrez Rivas, E. et al. (2009). "Chronic fatigue syndrome: aetiology, diagnosis and treatment.". BMC Psychiatry. 9 Suppl 1: S1. doi:10.1186/1471-244X-9-S1-S1. PMID 19857242.
- Systematic review of the comorbidity of irritable bowel syndrome with other disorders: What are the causes and implications? In: Gastroenterology, Volume 122, Issue 4, April 2002, Pages 1140-1156. Retrieved Retrieved June 20, 2011.
- Review article: fructose malabsorption and the bigger picture 2006, Wiley Online Library. Retrieved June 20, 2011.
- Nijrolder I, van der Windt D, de Vries H, van der Horst H (November 2009). "Diagnoses during follow-up of patients presenting with fatigue in primary care". CMAJ 181 (10): 683–7. doi:10.1503/cmaj.090647. PMC 2774363. PMID 19858240.
- EMedicine Health. "Fatigue Exams and tests" 2010-01-25.
- Fatigue — Information for Patients, U.S. National Cancer Institute
- Tiredness — Information leaflet from mental health charity The Royal College of Psychiatrists
- Is exhaustion an actual medical diagnosis? from Slate.com