Exhausting
Chronic: Chronic fatigue is a self-reported fatigue lasting at least 6 consecutive months. Chronic fatigue may be either persistent or relapsing. Chronic fatigue is a symptom of many chronic illnesses and of idiopathic chronic fatigue.

By effect: Fatigue can have significant negative impacts on quality of life. Profound and debilitating fatigue is the most common complaint reported among individuals with autoimmune disease, such as systemic lupus erythematosus, multiple sclerosis, type 1 diabetes, celiac disease, Myalgic Encephalomyelitis/chronic fatigue syndrome, and rheumatoid arthritis. Fatigue has been described by sufferers as 'incomprehensible' due to its unpredictable occurrence, lack of relationship to physical effort and different character as compared to tiredness. Fatigue that dissociates by quantum with disease activity represents a large health economic burden and unmet need to patients and to society.

WHO classification: The World Health Organization's ICD-11 classification
includes a category MG22 Fatigue (typically fatigue following exertion but sometimes may occur in the absence of such exertion as a symptom of health conditions), and many other categories where fatigue is mentioned as a secondary result of other factors. It does not include any fatigue-based psychiatric illness (unless it is accompanied by related psychiatric symptoms).
DSM-5 lists 'fatigue or loss of energy nearly every day' as one factor in diagnosing depression.

Treatment: Management may include review of factors and methods as explained below.

Cessation of medications causing fatigue: Taking of medications with side effects of contributing to fatigue may be ceased.

Medications to treat fatigue: The UK NICE recommends consideration of amantadine, modafinil and SSRIs for MS fatigue. Psychostimulants such as methylphenidate, amphetamines, and modafinil have been used in the treatment of fatigue related to depression, and medical illness such as chronic fatigue syndrome and cancer. They have also been used to counteract fatigue in sleep loss and in aviation.

Mental health tools: CBT can be useful for fatigue, including ME/CFS but is not included in NICE guidelines for ME/CFS treatment.

Other approaches: 

Avoidance of body heat: Fatigue in MS often correlates with relatively high endogenous body temperature.

Improved sleep: Improving sleep has been associated with reduced fatigue but only in small studies.

Intermittent fasting: A very small 2022 study found 40% reductions in fatigue categorisations after three months of 16:8 intermittent fasting.

Vagus nerve stimulation: A very small 2023 study of Sjogren's patients showed reductions in self-reported fatigue after 56 days of vagus nerve stimulation.

Qigong and Tai Chi: Qigong and Tai chi have been postulated as helpful to reduce fatigue, but the evidence is of low quality.

Prevalence: 2023 guidance stated fatigue prevalence is between 4.3% and 21.9%. Prevalence is higher in women than men.
A 2021 German study found that fatigue was the main or secondary reason for 10–20% of all consultations with a primary care physician.
A large study based on the 2004 Health and Retirement Study (HRS), a biennial longitudinal survey of US adults aged 51 and above, with mean age 65, found that 33% of women and 29% of men self-reported fatigue.

Possible purposes of fatigue: 

Body resource management purposes: Fatigue has been posited as a bio-psycho-physiological state reflecting the body's overall strategy in resource (energy) management. Fatigue may occur when the body wants to limit resource utilisation ("rationing") in order to use resources for healing (part of sickness behaviour) or conserve energy for a particular current or future anticipated need, including a threat.

Evolutionary purposes: It has been posited that fatigue had evolutionary benefits in making more of the body's resources available for healing processes, such as immune responses, and in limiting disease spread by tending to reduce social interactions.

See also: 

References: 

Further reading: Byung-Chul Han: Müdigkeitsgesellschaft. Matthes & Seitz, Berlin 2010, ISBN 978-3-88221-616-5. (Philosophical essay about fatigue as a sociological problem and symptom).
Danish edition: Træthedssamfundet. Møller, 2012, ISBN 978-87-994043-7-7.
Dutch edition: De vermoeide samenleving. van gennep, 2012, ISBN 978-94-6164-071-0.
Italian editions: La società della stanchezza. nottetempo, 2012, ISBN 978-88-7452-345-0.
Korean edition: 한병철 지음 | 김태환 옮김. Moonji, 2011, ISBN 978-89-320-2396-0.
Spanish edition: La sociedad del cansancio. Herder Editorial, 2012, ISBN 978-84-254-2868-5.

External links: 
Fatigue –  Information for Patients, U.S. National Cancer Institute