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Other namesExhaustion, weariness, tiredness, lethargy, languidness, languor, lassitude, listlessness
Specialty Internal medicine, Family practice, Psychiatry, Psychology

Fatigue is a feeling of tiredness. [1] It may be sudden or gradual in onset. It is a normal phenomenon if it follows prolonged physical or mental activity, and resolves completely with rest. However, it may be a symptom of a medical condition if it is prolonged, severe, progressive, or occurs without provocation.


Physical fatigue is the transient inability of muscles to maintain optimal physical performance, and is made more severe by intense physical exercise. [2] [3] [4] Mental fatigue is a transient decrease in maximal cognitive performance resulting from prolonged periods of cognitive activity. Mental fatigue can manifest as somnolence, lethargy, or directed attention fatigue. [5]

Fatigue and 'feelings of fatigue' are sometimes confused. [6] Unlike weakness, fatigue usually can be alleviated by periods of rest.


Physical fatigue

Physical fatigue, or muscle fatigue, is the temporary physical inability of muscles to perform optimally. The onset of muscle fatigue during physical activity is gradual, and depends upon an individual's level of physical fitness – other factors include sleep deprivation and overall health. Fatigue can be reversed by rest. [7] 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. [8] The central component of fatigue is triggered by an increase of the level of serotonin in the central nervous system. [9] During motor activity, serotonin released in synapses that contact motoneurons promotes muscle contraction. [10] 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. [11]

Muscle strength testing can be used to determine the presence of a neuromuscular disease, but cannot determine its cause. 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. [12]

People with multiple sclerosis experience a form of overwhelming lassitude or tiredness that can occur at any time of the day, for any duration, and that does not necessarily recur in a recognizable pattern for any given patient, referred to as "neurological fatigue". [13] [14]

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. [5] It can manifest as somnolence, lethargy, or directed attention fatigue. Decreased attention may also be described as a more or less decreased level of consciousness. [15] 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.[ citation needed ]

The perception of mental fatigue is believed to be modulated by the brain's reticular activating system (RAS).[ citation needed ]

Fatigue impacts a driver's reaction time, awareness of hazards around them and their attention. Drowsy drivers are three times more likely to be involved in a car crash and if they are awake over 20 hours, is the equivalent of driving with a blood-alcohol concentration level of 0.08%. [16]



Fatigue is a normal result of working, mental stress, overstimulation and understimulation, jet lag, active recreation, boredom, and lack of sleep.


Causes of acute fatigue include depression; chemical causes, such as dehydration, poisoning, low blood sugar, or mineral or vitamin deficiencies. Fatigue is different from drowsiness, where a patient feels that sleep is required.[ citation needed ]

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.


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. [17] Chronic fatigue is a symptom of many diseases and conditions. Some major categories of conditions that feature fatigue include:

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.


Inflammation has been linked to many types of fatigue. [18] Findings implicate neuroinflammation in the etiology of fatigue in autoimmune and related disorders. [18]


Minor dark circles, in addition to a hint of eye bags, a combination which is suggestive of sleep deprivation and/or mental fatigue. Dark circles (cropped).png
Minor dark circles, in addition to a hint of eye bags, a combination which is suggestive of sleep deprivation and/or mental fatigue.

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% of cases. [25]

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 anemia, 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. [26] Other tests may be chosen depending on the patient's social history, such as an HIV test or pregnancy test.

Comparison with sleepiness

Fatigue is generally considered a more long-term condition than sleepiness (somnolence). [27] Although sleepiness can be a symptom of a medical condition, it usually results from lack of restful sleep, or a lack of stimulation. [28] 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. [29] Both are often used interchangeably and even categorized under the description of 'being tired.' Fatigue is often described as an uncomfortable tiredness, whereas sleepiness is comfortable and inviting.


Fatigue can be quantitatively measured. Devices to measure medical fatigue have been developed by Japanese companies, among them Nintendo (cancelled). [30] Nevertheless, such devices are not in common use outside Japan.

See also

Related Research Articles

Sleep disorder Medical disorder of the sleep patterns of a person

A sleep disorder, or somnipathy, is a medical disorder of the sleep patterns of a person. Some sleep disorders are serious enough to interfere with normal physical, mental, social and emotional functioning. Polysomnography and actigraphy are tests commonly ordered for some sleep disorders.

Fibromyalgia Chronic disorder of unknown cause characterized by pain, stiffness, and widespread tenderness in muscles

Fibromyalgia (FM) is a medical condition characterized by chronic widespread pain and a heightened pain response to pressure. Other symptoms include tiredness to a degree that normal activities are affected, sleep problems and troubles with memory. Some people also report restless legs syndrome, bowel or bladder problems, numbness and tingling and sensitivity to noise, lights or temperature. Fibromyalgia is frequently associated with depression, anxiety and posttraumatic stress disorder. Other types of chronic pain are also frequently present.

Post-polio syndrome Human disease

Post-polio syndrome are latent symptoms of poliomyelitis (polio), occurring at about a 25% to 40% rate. It is a viral infection of the nervous system after the initial infection. Symptoms typically occur 15 to 30 years after an initial acute paralytic attack. Symptoms include decreasing muscular function or acute weakness with pain and fatigue. The same symptoms may also occur years after a nonparalytic polio (NPP) infection.

Encephalopathy means any disorder or disease of the brain, especially chronic degenerative conditions. In modern usage, encephalopathy does not refer to a single disease, but rather to a syndrome of overall brain dysfunction; this syndrome has many possible organic and inorganic causes.

Weakness is a symptom of a number of different conditions. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.

Somnolence is a state of strong desire for sleep, or sleeping for unusually long periods. It has distinct meanings and causes. It can refer to the usual state preceding falling asleep, the condition of being in a drowsy state due to circadian rhythm disorders, or a symptom of other health problems. It can be accompanied by lethargy, weakness, and lack of mental agility.

Hypersomnia is a neurological disorder of excessive time spent sleeping or excessive sleepiness. It can have many possible causes and can cause distress and problems with functioning. In the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), hypersomnolence, of which there are several subtypes, appears under sleep-wake disorders.

Exercise intolerance

Exercise intolerance is a condition of inability or decreased ability to perform physical exercise at what would be considered to be the normally expected level or duration. It also includes experiences of unusually severe post-exercise pain, fatigue, nausea, vomiting or other negative effects. Exercise intolerance is not a disease or syndrome in and of itself, but can result from various disorders.

Obstructive sleep apnea Obstruction of the upper airway during sleep

Obstructive sleep apnea (OSA) is the most common type of sleep apnea and is characterized by repeated episodes of complete or partial obstructions of the upper airway during sleep, despite the effort to breathe, and is usually associated with a reduction in blood oxygen saturation. In the Obstructive Sleep Apnea-Hypopnea Syndrome, the episodes of decreased breathing are called "hypopnea" and its definition requires a ≥30% drop in flow for 10 seconds or longer, associated with ≥3% oxygen desaturation. The episodes of breathing cessations are called "apneas" and to be defined, a ≥90% drop in flow for 10 seconds or longer must be assessed and associated with ≥3% oxygen desaturation, or an arousal.

Cataplexy is a sudden and transient episode of muscle weakness accompanied by full conscious awareness, typically triggered by emotions such as laughing, crying, or terror. Cataplexy affects approximately 70% of people who have narcolepsy, and is caused by an autoimmune destruction of hypothalamic neurons that produce the neuropeptide hypocretin, which regulates arousal and has a role in stabilization of the transition between wake and sleep states. Cataplexy without narcolepsy is rare and the cause is unknown.

Muscle weakness is a lack of muscle strength. The causes are many and can be divided into conditions that have either true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis. Muscle weakness can also be caused by low levels of potassium and other electrolytes within muscle cells. It can be temporary or long-lasting. The term myasthenia is from my- from Greek μυο meaning "muscle" + -asthenia ἀσθένεια meaning "weakness".

Neuromuscular disease neuropathy that affect the nerves that control the voluntary muscles

Neuromuscular disease is a broad term that encompasses many diseases and ailments that impair the functioning of the muscles, either directly, being pathologies of the voluntary muscle, or indirectly, being pathologies of nerves or neuromuscular junctions.

Medically unexplained physical symptoms are symptoms for which a treating physician or other healthcare providers have found no medical cause, or whose cause remains contested. In its strictest sense, the term simply means that the cause for the symptoms is unknown or disputed—there is no scientific consensus. Not all medically unexplained symptoms are influenced by identifiable psychological factors. However, in practice, most physicians and authors who use the term consider that the symptoms most likely arise from psychological causes. Typically, the possibility that MUPS are caused by prescription drugs or other drugs is ignored. It is estimated that between 15% and 30% of all primary care consultations are for medically unexplained symptoms. A large Canadian community survey revealed that the most common medically unexplained symptoms are musculoskeletal pain, ear, nose, and throat symptoms, abdominal pain and gastrointestinal symptoms, fatigue, and dizziness. The term MUPS can also be used to refer to syndromes whose etiology remains contested, including chronic fatigue syndrome, fibromyalgia, multiple chemical sensitivity and Gulf War illness.

The clinical descriptions of chronic fatigue syndrome (CFS) vary. Different agencies and scientific bodies have produced different guidelines to define the condition, with some overlap of symptoms between descriptions. Aspects of the condition are controversial, with disagreements over etiology, pathophysiology, treatment and naming between medical practitioners, researchers, patients and advocacy groups. Subgroup analysis suggests that, depending on the applied definition, the CFS population may represent a variety of conditions rather than a single disease entity.

Narcolepsy Sleep disorder that involves an excessive urge to sleep at inappropriate times

Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep-wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. About 70% of those affected also experience episodes of sudden loss of muscle strength, known as cataplexy. These experiences can be brought on by strong emotions. Less commonly, there may be vivid hallucinations or an inability to move while falling asleep or waking up. People with narcolepsy tend to sleep about the same number of hours per day as people without, but the quality of sleep tends to be lessened.

Idiopathic hypersomnia is a neurological disorder which is characterized primarily by excessive sleep and excessive daytime sleepiness (EDS). It has historically been rarely diagnosed and is often very difficult to diagnose at an early stage; it is usually a lifelong chronic disease, which is often debilitating. There is a very low level of public awareness of idiopathic hypersomnia, which often leads to stigma for those who suffer from it. There is currently no cure, but there are several off-label treatments, which are primarily FDA-approved narcolepsy medications.

Differential diagnoses of depression Differential diagnoses

Depression, one of the most commonly diagnosed psychiatric disorders, is being diagnosed in increasing numbers in various segments of the population worldwide. Depression in the United States alone affects 17.6 million Americans each year or 1 in 6 people. Depressed patients are at increased risk of type 2 diabetes, cardiovascular disease and suicide. Within the next twenty years depression is expected to become the second leading cause of disability worldwide and the leading cause in high-income nations, including the United States. In approximately 75% of completed suicides, the individuals had seen a physician within the prior year before their death, 45–66% within the prior month. About a third of those who completed suicide had contact with mental health services in the prior year, a fifth within the preceding month.

Chronic fatigue syndrome (CFS), also referred to as myalgic encephalomyelitis (ME), is a medical condition characterized by long-term fatigue, worsening symptoms after physical or mental activity, and unrefreshing sleep. Difficulty sitting and standing upright or problems with memory or thinking are also present. The condition often limits a person's ability to carry out ordinary daily activities. People's ability to function differs from that before the onset of the disease. Other symptoms may involve numerous body systems and are present in many of those affected.

Lithium toxicity

Lithium toxicity, also known as lithium overdose, is the condition of having too much lithium. Symptoms may include a tremor, increased reflexes, trouble walking, kidney problems, and a altered level of consciousness. Some symptoms may last for a year after levels return to normal. Complications may include serotonin syndrome.

Post-exertional malaise (PEM) is one of the main symptoms of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). PEM can be described as "a delayed and significant exacerbation of ME/CFS symptoms that always follows physical activity and often follows cognitive activity".


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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).

External resources