They discover why thinking too much causes fatigue

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Researchers have new evidence to explain why hard mental work causes tiredness, and according to their findings, the reason you feel

rather than drowsy from intense thinking is a functioning brain defense system.

Their studies, published in the journal Current Biology, show that when intense cognitive work continues for several hours,

accumulate in the part of the brain known as the

.

This, in turn,

, so that it shifts toward low-cost actions that don't require effort or waiting as cognitive fatigue sets in, the researchers explain.

"The most influential theories suggest that fatigue is a kind of

for us to stop doing whatever it is and dedicate ourselves to a more rewarding activity," explains Mathias Pessiglione, from the Pitié-Salpêtrière University in Paris (France). ).

"But our findings show that cognitive work causes a true functional alteration through the

, so fatigue would indeed be a signal that makes us stop working, but with a different purpose: to preserve the integrity brain function," he adds.

Pessiglione and his colleagues, including the study's first author

, wanted to understand what

really is .

While machines can compute continuously, the brain cannot.

They wanted to find out why, since they suspected that the reason had to do with the need to recycle potentially toxic substances that arise from neuronal activity.

To look for evidence of this, they used

(MRS) to monitor brain chemistry throughout a working day.

They studied two groups of people: those who needed to think a lot and those who had relatively easier cognitive tasks.

They observed signs of fatigue, including

, only in the group that did hard work.

Those in that group also showed in their choices a shift toward options that offered short-term rewards with little effort.

More importantly, they also had

at synapses in the brain's prefrontal cortex.

Together with previous evidence, the authors say this supports the idea that glutamate accumulation makes it more difficult to activate the prefrontal cortex, so that cognitive control is more difficult after a mentally hard day's work.

Given this certainty, the researchers believe that

of the brain's ability to think intensely.

"I'm afraid not, says Pessiglione. I would use the good old recipes:

. There is good evidence that glutamate is cleared from synapses during sleep," he says.

The researchers note that there may be other practical implications, such as monitoring prefrontal metabolites could help detect severe mental fatigue.

This ability could help adjust work schedules to avoid burnout.

He also advises avoiding making important decisions when you are tired.

In future studies, they hope to learn why the prefrontal cortex seems especially susceptible to glutamate buildup and fatigue.

They are also curious whether the same fatigue markers in the brain can predict recovery from illnesses, such as depression or cancer.

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