Hard physical labor naturally exhausts you, but what about hard mental labor? When you just sit around and think hard for hours, you get tired, too.

The reason you experience mental exhaustion (as opposed to drowsiness) after heavy thinking isn’t all in your head, according to recent research, which explains why this happens.

Their research, published in Current Biology today, demonstrates that prolonged, high-intensity cognitive exertion leads to a buildup of potentially hazardous wastes in the prefrontal cortex. 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.

“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,” explains Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. 

But the findings of the study show that “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.”

Pessiglione and his associates, including the study’s first author, Antonius Wiehler, aimed to define mental tiredness precisely. Machines can do math all the time, but the brain can’t. They were curious as to why. They thought that the reason had to do with the need to recycle substances that could be harmful that are made when neurons fire.

They 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.

Only in the group engaged in heavy work did they notice indicators of fatigue, such as decreased pupil dilation 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.

So, is there a way to get around the fact that our brains can’t think too hard?

“Not really, I’m afraid,” Pessiglione adds. “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

There may be other real-world effects. The researchers suggest that monitoring prefrontal metabolites is one way to detect 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.

In future studies, they want to find out why the prefrontal cortex seems to be especially vulnerable 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.

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