Fatigue

===By type===

====Uni- or multi-dimensional====
Fatigue can be seen as a uni-dimensional phenomenon that influences different aspects of human life.<ref>{{cite journal |author1=Roald Omdal|author2=Svein Ivar Mellgren|author3= Katrine Brække Norheim|title=Pain and fatigue in primary Sjögren's syndrome |journal=Rheumatology |date=July 2021 |volume=6 |issue=7 |pages=3099–3106 |doi=10.1093/rheumatology/kez027|pmid=30815693 }}</ref><ref>{{Cite journal|title=Importance of fatigue and its measurement in chronic liver disease|first1=Lynn H|last1=Gerber|first2=Ali A|last2=Weinstein|first3=Rohini|last3=Mehta|first4=Zobair M|last4=Younossi|date=July 28, 2019|journal=World Journal of Gastroenterology|volume=25|issue=28|pages=3669–3683|doi=10.3748/wjg.v25.i28.3669|doi-access=free |pmid=31391765|pmc=6676553}}</ref> It can be multi-faceted and broadly defined, making understanding the causes of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases.<ref name="pmid31447842">{{cite journal | pmid=31447842 | year=2019 | last1=Zielinski | first1=M. R. | last2=Systrom | first2=D. M. | last3=Rose | first3=N. R. | title=Fatigue, Sleep, and Autoimmune and Related Disorders | journal=Frontiers in Immunology | volume=10 | page=1827 | doi=10.3389/fimmu.2019.01827 | pmc=6691096 | doi-access=free }}</ref>

A 2021 review considered that different "types/subsets" of fatigue may exist and that patients normally present with more than one such "type/subset". These different "types/subsets" of fatigue may be different dimensions of the same symptom, and the relative manifestations of each may depend on the relative contribution of different mechanisms. Inflammation may be the root causal mechanism in many cases.<ref name="pmid34599320" />

==== Physical ====
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.<ref name="WebMD">{{cite news|title=Weakness and fatigue|newspaper=Webmd|url=http://www.webmd.com/a-to-z-guides/weakness-and-fatigue-topic-overview|publisher=Healthwise Inc.|access-date=2 January 2013|archive-date=30 December 2012|archive-url=https://web.archive.org/web/20121230140117/http://www.webmd.com/a-to-z-guides/weakness-and-fatigue-topic-overview|url-status=live}}</ref> 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.<ref>{{cite journal | vauthors = Gandevia SC | title = Spinal and supraspinal factors in human muscle fatigue | journal = Physiological Reviews | volume = 81 | issue = 4 | pages = 1725–1789 | date = October 2001 | pmid = 11581501 | doi = 10.1152/physrev.2001.81.4.1725 }}</ref> The central component of fatigue is triggered by an increase of the level of [[serotonin]] in the central nervous system.<ref name="pmid10919962">{{cite journal | vauthors = Davis JM, Alderson NL, Welsh RS | title = Serotonin and central nervous system fatigue: nutritional considerations | journal = The American Journal of Clinical Nutrition | volume = 72 | issue = 2 Suppl | pages = 573S–578S | date = August 2000 | pmid = 10919962 | doi = 10.1093/ajcn/72.2.573S | doi-access = free }}</ref> During motor activity, serotonin released in synapses that contact [[motor neuron]]s promotes [[muscle contraction]].<ref>{{cite journal | vauthors = Perrier JF, Delgado-Lezama R | title = Synaptic release of serotonin induced by stimulation of the raphe nucleus promotes plateau potentials in spinal motoneurons of the adult turtle | journal = The Journal of Neuroscience | volume = 25 | issue = 35 | pages = 7993–7999 | date = August 2005 | pmid = 16135756 | pmc = 6725458 | doi = 10.1523/JNEUROSCI.1957-05.2005 }}</ref> 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 [[Action potential|nerve impulse]] initiation and thereby muscle contraction are inhibited.<ref>{{cite journal | vauthors = Cotel F, Exley R, Cragg SJ, Perrier JF | title = Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 12 | pages = 4774–4779 | date = March 2013 | pmid = 23487756 | pmc = 3607056 | doi = 10.1073/pnas.1216150110 | doi-access = free | bibcode = 2013PNAS..110.4774C }}</ref>

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.<ref>{{cite journal | vauthors = Enoka RM, Duchateau J | title = Muscle fatigue: what, why and how it influences muscle function | journal = The Journal of Physiology | volume = 586 | issue = 1 | pages = 11–23 | date = January 2008 | pmid = 17702815 | pmc = 2375565 | doi = 10.1113/jphysiol.2007.139477 | author-link = Roger M. Enoka }}</ref>

==== Mental ====  <!--Linked from [[Template:Mental processes]]-->

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.<ref name="Marcora 2009 857–864" /> 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]], though the validity of the concept is disputed. 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.<ref>{{cite journal | vauthors = Lin H, Saunders B, Friese M, Evans NJ, Inzlicht M | title = Strong Effort Manipulations Reduce Response Caution: A Preregistered Reinvention of the Ego-Depletion Paradigm | journal = Psychological Science | volume = 31 | issue = 5 | pages = 531–547 | date = May 2020 | pmid = 32315259 | pmc = 7238509 | doi = 10.1177/0956797620904990 }}</ref>

Decreased attention can also be described as a more or less decreased [[level of consciousness]].<ref>{{cite book|title=Difficult Diagnosis 2| vauthors  = Giannini AJ |publisher=W.B. Saunders Co.|year=1991|isbn=978-0-7216-3481-4| veditors = Taylor RB |location= Philadelphia|page=156|chapter=Fatigue, Chronic|oclc=954530793}}</ref> 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.<ref name="pmid29355911">{{cite journal |vauthors=Possin KL, Moskowitz T, Erlhoff SJ, Rogers KM, Johnson ET, Steele NZ, Higgins JJ, Stiver J, Alioto AG, Farias ST, Miller BL, Rankin KP |title=The Brain Health Assessment for Detecting and Diagnosing Neurocognitive Disorders |journal=J Am Geriatr Soc |volume=66 |issue=1 |pages=150–156 |date=January 2018 |pmid=29355911 |pmc=5889617 |doi=10.1111/jgs.15208 |url=}}</ref><ref name="pmid30884965">{{cite journal |vauthors=Menzies V, Kelly DL, Yang GS, Starkweather A, Lyon DE |title=A systematic review of the association between fatigue and cognition in chronic noncommunicable diseases |journal=Chronic Illn |volume=17 |issue=2 |pages=129–150 |date=June 2021 |pmid=30884965 |pmc=6832772 |doi=10.1177/1742395319836472 |url=}}</ref><ref name="pmid38079429">{{cite journal |vauthors=Elliott TR, Hsiao YY, Randolph K, Urban RJ, Sheffield-Moore M, Pyles RB, Masel BE, Wexler T, Wright TJ |title=Efficient assessment of brain fog and fatigue: Development of the Fatigue and Altered Cognition Scale (FACs) |journal=PLOS ONE |volume=18 |issue=12 |pages=e0295593 |date=2023 |pmid=38079429 |pmc=10712873 |doi=10.1371/journal.pone.0295593 |doi-access=free |bibcode=2023PLoSO..1895593E |url=}}</ref><!--It would also be interesting to read more about the ways in which the [[neurocognitive]] deficits of caused by fatigue differ from those caused by tiredness-->

The [[perception]] of mental fatigue is believed to be modulated by the brain's [[reticular activating system]] (RAS).<ref name="pmid24926625">{{cite journal |vauthors=Ishii A, Tanaka M, Watanabe Y |title=Neural mechanisms of mental fatigue |journal=Rev Neurosci |volume=25 |issue=4 |pages=469–79 |date=2014 |pmid=24926625 |doi=10.1515/revneuro-2014-0028 |url=}}</ref><ref name="pmid28690375">{{cite journal |vauthors=Garcia-Rill E, Virmani T, Hyde JR, D'Onofrio S, Mahaffey S |title=Arousal and the control of perception and movement |journal=Curr Trends Neurol |volume=10 |issue= |pages=53–64 |date=2016 |pmid=28690375 |pmc=5501251 |doi= |url=}}</ref><ref name="pmid31216564">{{cite journal |vauthors=Jones BE |title=Arousal and sleep circuits |journal=Neuropsychopharmacology |volume=45 |issue=1 |pages=6–20 |date=January 2020 |pmid=31216564 |pmc=6879642 |doi=10.1038/s41386-019-0444-2 |url=}}</ref><ref name="pmid37155119">{{cite journal |vauthors=Taran S, Gros P, Gofton T, Boyd G, Briard JN, Chassé M, Singh JM |title=The reticular activating system: a narrative review of discovery, evolving understanding, and relevance to current formulations of brain death |journal=Can J Anaesth |volume=70 |issue=4 |pages=788–795 |date=April 2023 |pmid=37155119 |pmc=10203024 |doi=10.1007/s12630-023-02421-6 |url=}}</ref><ref name="pmid31751025">{{cite book|pmid=31751025|title=Neuroanatomy, Reticular Activating System|date=2024 | vauthors = Arguinchona JH, Tadi P }}</ref>

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 being awake over 20 hours is the equivalent of driving with a blood-alcohol concentration level of 0.08%.<ref>{{cite web |title=Drowsy Driving is Impaired Driving |url=https://www.nsc.org/road-safety/safety-topics/fatigued-driving |website=National Safety Council |access-date=31 January 2019 |archive-date=1 February 2019 |archive-url=https://web.archive.org/web/20190201014026/https://www.nsc.org/road-safety/safety-topics/fatigued-driving |url-status=live }}</ref>

====Neurological fatigue====

People with [[multiple sclerosis]] experience a form of overwhelming 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" or "lassitude".<ref name="pmid32672087">{{cite journal | vauthors = Hubbard AL, Golla H, Lausberg H | title = What's in a name? That which we call ''Multiple Sclerosis Fatigue'' | journal = Multiple Sclerosis | volume = 27 | issue = 7 | pages = 983–988 | date = June 2021 | pmid = 32672087 | pmc = 8142120 | doi = 10.1177/1352458520941481 }}</ref><ref>{{cite journal | vauthors = Mills RJ, Young CA, Pallant JF, Tennant A | title = Development of a patient reported outcome scale for fatigue in multiple sclerosis: The Neurological Fatigue Index (NFI-MS) | journal = Health and Quality of Life Outcomes | volume = 8 | page = 22 | date = February 2010 | pmid = 20152031 | pmc = 2834659 | doi = 10.1186/1477-7525-8-22 | doi-access = free }}</ref><ref>{{Cite web|url=https://www.tiktok.com/@msfighter101/video/7242002519809854763|title=TikTok - Make Your Day|website=www.tiktok.com|access-date=2024-03-26|archive-date=2024-03-26|archive-url=https://web.archive.org/web/20240326105609/https://www.tiktok.com/@msfighter101/video/7242002519809854763|url-status=live}}</ref>

People with [[autoimmune diseases]] including inflammatory rheumatic diseases such as [[rheumatoid arthritis]], [[psoriatic arthritis]] and primary [[Sjögren's syndrome]], experience similar fatigue.<ref name="pmid34599320"/><ref name="pmid31447842"/>
Attempts have been made to isolate causes of [[central nervous system fatigue]].