Fatigue is described as the lack of energy and motivation (both physical and mental). It has to be distinguish from drowsiness, which is a term that describes the need to sleep.

Medically, fatigue is a non-specific symptom, which means that it has many possible causes and accompanies many different conditions. 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.

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. Chronic blood loss frequently results in fatigue, as do other conditions that cause anemia. 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. 

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. 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. Muscle strength testing can be used to determine the presence of a neuromuscular disease, but cannot determine its etiology. Additional testing, such as electromyography (an electrodiagnostic medicine technique for evaluating and recording the electrical activity produced by skeletal muscles), can provide diagnostic information, but information gained from muscle strength testing alone is not enough to diagnose most neuromuscular disorders.

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. 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 (a temporary episode of sleep which may last for a fraction of a second or up to 30 seconds). However, objective cognitive testing can be used to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.

Chronic fatigue syndrome (CFS) is a medical condition characterized by long-term fatigue and other symptoms that limit a person's ability to carry out ordinary daily activities. Quality of life of persons with CFS can be compromised. 

Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood. The fatigue of CFS is not due to ongoing exertion, is not much relieved by rest, and is not due to any other medical condition. Diagnosis is based on a person's signs and symptoms. 

Evidence suggests that cognitive behavioral therapy and a gradual increase in activity suited to individual capacity can be beneficial in some cases. 

CFS occurs more often in women than in men, and is less common among children and adolescents. There is agreement that CFS has a negative effect on health, happiness and productivity but there is also controversy over many aspects of the disorder. Physicians, researchers and patient advocates promote different names and diagnostic criteria, while evidence for proposed causes and treatments is often contradictory or of low quality.

Fatigue can accompany several diseases of reproductive system such as endometrial hyperplasia, tubal ligation, ovarian cancer, orchitis, prostatitis and anorexia nervosa.

Endometrial hyperplasia

Endometrial hyperplasia is a condition of excessive proliferation of the cells inner lining of the uterus. One of the symptom of endometrial hyperplasia is heavy beeding. Blood loss may be significant enough to cause anemia, with associated symptoms of fatigue, dizziness, and moodiness.

Tubal ligation

Tubal ligation (a surgical procedure for sterilization in which a woman's fallopian tubes are clamped and blocked or severed and sealed) is the most popular family planning method worldwide. While its benefits, such as effectiveness in protecting against pregnancies, minimal need for long-term follow-up and low side-effects profile are well documented. But several symptoms are linked to psychosocial issues. Fatigue, regret, undifferentiated complaints are some of them.

Ovarian cancer

Cancer Related Fatigue (CRF) is one of the most disabling and distressing symptoms affecting physical and emotional wellbeing of patients diagnosed with cancer or receiving its treatment. Chronic CRF has been reported to be also associated with poor quality of life, and is also associated with considerable psychological distress.

Orchitis, prostatitis

These two diagnosis have something in common. Both are inflammatory diseases, and both can lead to chronicity followed by chronic fatigue syndrom.

Anorexia nervosa

Anorexia nervosa is an eating disorder characterized by extremely low body weight, fear of gaining weight or distorted perception of body image. Central to the pathogenesis of the refeeding syndrome is a weakened cardiopulmonary system, electrolytes abnormalities, hepatic dysfunction, liver hypoperfusion and failure. This all condition can lead to fatigue.

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