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Why you get exhausted when you think hard

The reason you experience mental exhaustion rather than drowsiness from heavy thought isn't all in your brain, according to new research that sheds light on the matter.

Their research, which was published in August 2022 in Current Biology, demonstrates that prolonged, high cognitive exertion can lead to the buildup of potentially hazardous byproducts in the prefrontal cortex of the brain. The researchers explain that once cognitive weariness sets in, this in turn affects your ability to manage judgments, causing you to gravitate toward low-cost acts that don’t require waiting or effort.

Pessiglione and his associates, including the study’s initial author, Antonius Wiehler, aimed to define mental tiredness precisely.

The brain cannot continually calculate, whereas computers can. They were curious as to why. They hypothesised that the necessity to recycle potentially harmful byproducts of brain activity was the cause.

Researchers monitored the chemistry of the brain during a workday using magnetic resonance spectroscopy (MRS) to search for proof of this. They studied two groups of people: those who had challenging cognitive tasks to complete and those who had relatively simple cognitive tasks. Just in the group engaged in heavy work did they notice indicators of weariness, such as decreased pupillary dilation. Additionally, those in that group displayed a shift in their preferences toward options that promised benefits with minimal work and a short wait time. Importantly, they also had larger glutamate concentrations in the prefrontal cortex of the brain’s synapses. The authors claim that data supports the theory that glutamate accumulation makes additional prefrontal cortex activation more expensive, making it harder to maintain cognitive control after a mentally taxing workday.

“Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity,” says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. “But our findings show that cognitive work results in a true functional alteration — accumulation of noxious substances — so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

There might be more useful implications.

For instance, monitoring prefrontal metabolites, the researchers claim, may assist in spotting extreme mental tiredness. Such a skill could aid in modifying work schedules to prevent burnout. He also counsels against making significant judgments when you’re sleep deprived.

Future research will attempt to explain why the prefrontal brain appears to be particularly prone to glutamate buildup and tiredness. Additionally, they are interested in whether the same brain indicators of weariness might forecast a person’s prognosis for diseases like depression or cancer.

Why you get exhausted when you think hard

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