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Near the end of his first series of chess matches against IBM\u2019s Deep Blue computer in 1996, the Russian grandmaster Garry Kasparov lamented what he saw as an unfair disadvantage: \u201cI\u2019m really tired. These games took a lot of energy. But if I play a normal human match, my opponent would also be exhausted.\u201d<\/p>\n
Why thinking hard makes us feel tired<\/p>\n<\/p>\n
Whereas machine intelligence can keep running as long as it has a power supply, a human brain will become fatigued \u2014 and you don\u2019t have to be a chess grandmaster to understand the feeling. Anyone can end up drained after a long day of work, at school or juggling the countless decisions of daily life. This mental exhaustion can sap motivation, dull focus and erode judgement. It can raise the odds of careless mistakes. Especially when combined with sleep loss or circadian disruption, cognitive fatigue can also contribute to deadly medical errors and road traffic accidents.<\/p>\n
It was partly Kasparov\u2019s weary comments that inspired Mathias Pessiglione, a cognitive neuroscientist and research director at the Paris Brain Institute, to study the tired brain. He wanted to know: \u201cWhy is this cognitive system prone to fatigue?\u201d<\/p>\n
Researchers and clinicians have long struggled to define, measure and treat cognitive fatigue \u2014 relying mostly on self-reports of how tired someone says they feel. Now, however, scientists from across disciplines are enlisting innovative experimental approaches and biological markers to probe the metabolic roots and consequences of cognitive fatigue.<\/p>\n
The efforts are getting a boost in attention and funding in large part because of long COVID, which afflicts roughly 6 in every 100 people after infection with the coronavirus SARS-CoV-2, says Vikram Chib, a biomedical engineer at Johns Hopkins University in Baltimore, Maryland. \u201cThe primary symptom of long COVID is fatigue,\u201d says Chib. \u201cI think that has opened a lot of people\u2019s eyes.\u201d<\/p>\n
Chib and others hope that a fundamental understanding of cognitive fatigue will help the billions of people who face it from time to time, as well as the tens of millions who carry it as a more extreme and chronic companion. As well as being common in long COVID, debilitating fatigue is a symptom of chronic fatigue syndrome, also known as myalgic encephalomyelitis (ME\/CFS), post-traumatic stress disorder, multiple sclerosis, depression and Parkinson\u2019s disease. Extreme mental exhaustion can also follow cancer treatment, head injury, stroke or exposure to certain toxins.<\/p>\n
\u201cFatigue is a really big problem,\u201d says Chib. \u201cWe really need to figure this out \u2014 how to study it and how to intervene.\u201d<\/p>\n
At the start of a chess match, a professional player might rely on well-rehearsed openings. \u201cThe first five, six or seven moves can be done without thinking,\u201d says Pessiglione. But when there\u2019s an unfamiliar position on the board, the player no longer has a ready routine. They are \u201crequired to think\u201d, he says. The same is true for a driver who turns on to an unfamiliar street. On roads they\u2019ve been down hundreds of times, mental autopilot can kick in. But if they take an unknown route, Pessiglione says, the demands on the brain intensify.<\/p>\n
Cognitive control is the term that scientists give to this effortful directing and regulating of thought. Over time, as a leading theory goes, maintaining control becomes costly for the brain, and fatigue emerges. Scientists aren\u2019t entirely sure why. Some think it is related to how cells cope when their energy supply is strained; others point to a build-up of toxins from neural activity1. But researchers tend to agree that the sensation of fatigue is protective \u2014 a warning that the brain is nearing a physiological boundary and it\u2019s time to rest.<\/p>\n
During chess matches in 1996 and 1997, Russian grandmaster Garry Kasparov grew tired, unlike his opponent, IBM\u2019s Deep Blue computer.<\/span>Credit: Stan Honda\/AFP via Getty<\/span><\/p>\n Molecular players across several brain regions might be involved, scientists say. Researchers have found potential links between cognitive fatigue and changing levels of metabolites such as glucose and lactate, neurochemical messengers such as glutamate and adenosine, and a protein involved in learning and memory called brain-derived neurotrophic factor2. Even amyloid-\u03b2, a protein fragment associated with Alzheimer\u2019s disease, might contribute by disrupting synapses, interfering with the clearance of glutamate or increasing neuroinflammation3. But it is uncertain what is a marker and what is a cause.<\/p>\n Cognitive neuroscientist Clay Holroyd at Ghent University in Belgium is among the researchers who favour the toxic-build-up theory of cognitive fatigue. This idea has been gaining traction over the past five years or so, although it\u2019s not yet clear what the waste metabolite is. He likens the sensation of fatigue to pain: both have a role in protecting the body from accumulating damage.<\/p>\n But even if a person ignores the sensation at first, he says, there\u2019s little risk of real metabolic harm in most cases. \u201cWe have an automatic fail-safe,\u201d he says. \u201cYou can work really, really, really hard for a time, but eventually you\u2019re going to need to go to sleep.\u201d Although taking a break from a task can offer temporary relief, sleep has a much more restorative role. Sleep, especially slow-wave deep sleep, acts as the brain\u2019s nightly maintenance. It clears out metabolic debris and recalibrates circuits and cells so that they can best use energy reserves.<\/p>\n Conventionally, cognitive fatigue has been quantified either by asking a person to report their own levels of fatigue or by detecting a change in their performance on working-memory tests or other tasks. These are not great measurements, scientists say. Drops in performance can be obscured by factors such as motivation, boredom and frustration. Performance can also be offset by training, such as when a chess player automates a sequence of moves.<\/p>\n Self-reports, meanwhile, are subjective and unreliable. \u201cPeople are terrible judges of their own fatigue,\u201d says Daniel Forger, a researcher in computational medicine at the University of Michigan in Ann Arbor, who is investigating new ways to assess cognitive fatigue.<\/p>\n Long-COVID research just got a big funding boost: will it find new treatments?<\/p>\n<\/p>\n To better understand fatigue, Pessiglione, Chib and other researchers are trying to bridge an understanding of its biochemical workings with how it affects motivation4. The current hypothesis: cognitive fatigue arises from metabolic changes in parts of the brain that are responsible for cognitive control. And those changes, whether resulting from depleted energy stores or amassed waste, alter how brain circuits weigh the costs and benefits of exerting mental effort \u2014 nudging decisions towards easier options that are more immediately rewarding1.<\/p>\n In a 2022 study5, Pessiglione and his team simulated a workday by asking otherwise healthy participants to spend several hours on either easy or hard versions of the same cognitive tasks. In one task, participants viewed letters appearing one after another on a screen and had to decide whether each new letter matched one a certain number of letters earlier. Recalling whether the letter on screen matched the third letter back, for example, would be much more difficult than recalling whether it matched the first letter back.<\/p>\n After this simulated workday, participants made choices between smaller immediate rewards and larger delayed ones. Those who had completed the harder tasks were more likely to opt for instant gratification. That preference was also consistent with greater accumulation of glutamate \u2014 one of the metabolites suspected to build up with cognitive exertion \u2014 in the lateral prefrontal cortex. This brain region is involved in executive functions, such as working memory and decision-making, and has been found to have lower activity after a hard day of simulated work6.<\/p>\n Matthew Apps, a cognitive neuroscientist at the University of Birmingham, UK, says that the dynamics of dopamine \u2014 which interacts closely with adenosine, glutamate and other metabolic players in the brain \u2014 might help to explain the connection between neurometabolic strain and the experience of fatigue. Because dopamine enhances the perceived value of rewards, it typically boosts motivation to invest effort. He proposes that sustained effort might cause dopamine levels to drop, leaving people less willing to work for the same pay-off.<\/p>\n Together, these and similar lines of research are starting to reveal how levels of these molecules and measures of brain activity align with fatigue.<\/p>\n The bottom line, says Chib, is that there is \u201csomething happening with the chemistry of your brain\u201d that changes your calculation on whether you should exert cognitive effort. He says that baseline chemistry will differ between people: \u201cYou and I might have different concentrations of neurotransmitters in our brain, and that inherent difference could make me feel very tired, whereas you\u2019re able to persist.\u201d<\/p>\n First proposed blood test for chronic fatigue syndrome: what scientists think<\/p>\n<\/p>\n These, or similar differences in chemistry, might help to explain chronic and other extreme fatigue. In people with long COVID or ME\/CFS, a small mental task can feel as intimidating as performing brain surgery.<\/p>\n For Ana Lia Tamariz, an artist and health-and-wellness coach in Miami, Florida, who has ME\/CFS, simply listening to music or reading a book often requires immense effort. \u201cSometimes, I cannot read one more word,\u201d she says. \u201cImagine coming out of anaesthesia from a surgical procedure groggy. Imagine you never exit that state.\u201d<\/p>\n For her, the cognitive and physical fatigue can be hard to separate. Any cognitively demanding task can make her physically drained, she says. And anything physically demanding can leave her mentally spent. Tamariz says she is constantly doing the calculation: \u201cIs this worth my energy?\u201d<\/p>\n Research supports the idea that some shared mechanisms might drive both physical and cognitive fatigue, and that the two interact7. \u201cIf you run a marathon, yes, there is physical fatigue in your body,\u201d says Apps. \u201cBut by God, you\u2019re going to feel all kinds of cognitive fatigue as well from the strain and the concentration that\u2019s required to maintain your performance.\u201d<\/p>\n The reverse also seems to be true. In a preprint from last year, Chib\u2019s team found that otherwise healthy participants were less willing to exert physical effort after performing a mentally taxing task8.<\/p>\n Other researchers are studying the roles of stress, sleep, circadian rhythms and inflammation in cognitive fatigue, as well as the consequences. When the brain doesn\u2019t get restorative sleep, small clusters of neurons can go briefly offline. These local sleep-like episodes can cause momentary lapses in attention and other slips in performance.<\/p>\n","protected":false},"excerpt":{"rendered":" Near the end of his first series of chess matches against IBM\u2019s Deep Blue computer in 1996, the Russian grandmaster Garry Kasparov lamented what he saw as an unfair disadvantage: \u201cI\u2019m really tired. These games took a lot of energy. But if I play a normal human match, my opponent would also be exhausted.\u201d Why<\/p>\n","protected":false},"author":1,"featured_media":36705,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[58],"tags":[2121,20445,8026,1031,5265,6074,8402],"class_list":{"0":"post-36704","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-brain","9":"tag-discovering","10":"tag-fatigue","11":"tag-mental","12":"tag-researchers","13":"tag-roots","14":"tag-tired"},"_links":{"self":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/36704","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=36704"}],"version-history":[{"count":0,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/36704\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/media\/36705"}],"wp:attachment":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=36704"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=36704"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=36704"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Seeking better measures<\/h2>\n