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Chapters

0:00 Introduction
0:24 You may be anaemic
0:51 Your thyroid may be underactive
1:15 Diabetes
1:32 A weak heart
1:51 Fibromyalgia
2:11 Arthritis
2:24 Mental illness
2:29 How to deal with it


Fatigue describes a state of tiredness that does not resolve with rest or sleep.[1] In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve after rest or sleep, or occurs independently of physical or mental exertion, it may be a symptom of a medical condition that may become severe or progressive.[2]

Fatigue can be a feature of a mental disorder such as depression; may be associated with conditions of chronic pain such as fibromyalgia; it may also feature in conditions of chronic low-level inflammation, and be a disease-related symptom in many other conditions.[3] Fatigue often has no known cause, and is recognised as being very complex in nature.[4] Fatigability describes a susceptibility to fatigue.[5]

Physical fatigue results from muscle fatigue brought about by intense physical activity.[6][7][8] Mental fatigue results from prolonged periods of cognitive activity which impairs cognitive ability. Mental fatigue can manifest as sleepiness, lethargy, or directed attention fatigue.[9] Mental fatigue can also impair physical performance.[10] 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.[11] 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, and can be reversed by rest.[12] The central component of fatigue is triggered by an increase of the level of serotonin in the central nervous system.[13] During motor activity, serotonin released in synapses that contact motor neurons promotes muscle contraction.[14] During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extrasynaptic receptors located on the axonal initial segment of motor neurons with the result that nerve impulse initiation and thereby muscle contraction are inhibited.[15]

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.[16]

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", and often as "multiple sclerosis fatigue".[17][18]
Mental

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.[9] It can manifest as somnolence, lethargy, directed attention fatigue, or disengagement. Research also suggests that mental fatigue is closely linked to the concept of ego depletion. For example, one pre-registered study of 686 participants found that after exerting mental effort, people are likely to disengage and become less interested in exerting further effort.[19] Decreased attention can also be described as a more or less decreased level of consciousness.[20] 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]