Why thinking really hard can feel like running a mile, scientifically speaking

Remember a time when you stared at your screen for 10 hours to finish a last-minute report for work, a 2,000-word essay on a book you’ve never read, or some other sort of mental marathon. At the end of it all, you probably felt the need to disassociate yourself from the world because your brain had turned to Jell-O.

We call this feeling mental fatigue — it’s not exactly that we feel sleepy, but our minds are weak and it becomes really difficult to do more complex reasoning than we already have. If we tried, we’d just be sick.

Here is the good news.

This flabby brain sensation is probably not just in our heads. According to a study published Thursday in the journal Current Biology, intense and prolonged cognitive activity literally causes potentially toxic by-products like an amino acid called glutamate to build up in our brains. These byproducts are believed to adjust our decision-making and prompt us to stop thinking so hard and turn to more relaxing and less stressful activities. And this may be the human body’s way of protecting itself from burnout.

“Influential theories suggest that fatigue is a kind of illusion concocted by the brain to make us stop whatever we’re doing and turn to a more rewarding activity,” said Mathias Pessiglione of Pitié-Salpêtrière University in France. , lead author of the study, said in a press release. “But our findings show that cognitive work leads to real functional impairment — accumulation of harmful substances — so fatigue could be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain function.”

“Even professional chess players start making mistakes, usually after 4-5 hours of play, which they wouldn’t if they were well rested,” the study authors write.

Pessiglione and his fellow researchers came to their conclusion after studying two groups of people with a technique called magnetic resonance spectroscopy, which measures biochemical changes in the brain. The first group was given difficult cognitive tasks, such as those involving stressful decisions related to the economy. The second had to perform much easier activities, such as identifying vowels and consonants with sufficient pause time between each question.

The team’s results showed that the group that had to think a lot more showed reduced pupil dilation and higher levels of glutamate in the brain. prefrontal cortex, the part that influences things like cognitive flexibility, attention, decision making and impulse control.

This then led the researchers to look at other relevant brain scan data and conclude that super-hard thinking likely leads to a buildup of glutamate in the brain, making it harder to activate our prefrontal cortex and therefore hinders our cognitive control and other prefrontal functions. Notably, however, the study cautions against viewing these findings as causal, stating that “our results are only correlational and cannot be taken as evidence that what limits cognitive control effort is the need to prevent the accumulation of glutamate”.

To confirm one way or another, further testing is needed. “Nevertheless,” the study writes, “glutamate regulation has been reported as a critical component of the brain’s energy budget and discussed as a potential source of cognitive fatigue.”

OK, so what’s the solution, you ask?

Unfortunately, according to Pessiglione, there isn’t, although the researcher suggests that “I would use good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

In other words, we might want to consider our mental activity the same way we would consider our physical activity. To climb a mountain, it’s usually best not to sprint, but rather to walk regularly – with a few breaks for food, water, and even a good night’s sleep.

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