Even chess grandmasters get tired in the head after four to five hours.

And so it is with anyone who has to work mentally hard: At some point during the day, concentration wanes until you end up feeling what neuroscientists, psychologists and behavioral scientists call mental exhaustion.

In principle, continuous top cognitive performance can be just as tiring as heavy physical activity, even if the reasons for it are different.

As far as brain work is concerned, French researchers may now be closer to an answer as to why intensive, long thinking so exhausts the brain.

They showed in a brain scanner that permanent mental stress leads to tangible biochemical disturbances in the forebrain.

Joachim Müller-Jung

Editor in the feuilleton, responsible for the "Nature and Science" department.

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The scientists led by Antonius Wiehler from the University of Pitié-Salpêtrière in Paris subjected two dozen test subjects to more than six hours of puzzles in the brain scanner and at the same time analyzed the brain activities and special metabolic products using magnetic resonance spectrography.

In addition, they had to make purchasing decisions that were presented to them at intervals.

For comparison, a second group of 16 people was occupied with tasks for just as long, but they were easy to solve.

The bottom line after the comparison was the realization that hard mental work obviously changes the chemistry of the brain, but this ultimately also provides the redeeming cognitive evasive maneuver - namely dealing with more relaxing things.

The increase in the messenger substance glutamate was noticeable in the brain images and metabolic analyzes by imaging.

This substance accumulated particularly in the forebrain, where one of the neuronal switching points for our decisions is located.

It has been found in amounts that neuroscientists believe makes it potentially poisonous to the mind.

Glutamate is known to be a vital brain messenger.

However, it is normally kept in a sensitive balance by the cell metabolism and rendered harmless accordingly.

This molecule also has other metabolic functions in the brain - such as nitrogen balance and protein production.

Previous studies had already shown that mental stress noticeably increases the glutamate concentration.

The researchers now interpret their results to mean that glutamate accumulates disproportionately in the synapses of the forebrain – the prefrontal cortex – during long-term cognitive stress.

This, in turn, revs up the glutamate metabolism in such a way that important cognitive functions in the forebrain, such as decision-making, suffer.

The same region then governs through neuronal evasive maneuvers.

At a certain point one is obviously unable to concentrate on the task at hand.

Thinking is then incredibly difficult, the brain gives way to other, cognitively less demanding mental activities.

In other words: the mind needs a day off.

According to this idea, the glutamate balance in the brain must be restored overnight or during longer recovery periods.

That's the theory.

In fact, the glutamate accumulations described in "Current Biology" and the corresponding changes in behavior have so far only been correlations.

A causal connection has not yet been proven.

To do this, you would have to carry out targeted experiments with different amounts of glutamate.

In addition, the focus of the investigation was placed almost entirely on glutamate.

The extent to which other brain messenger substances or bioelectrical connections play a role in cognitive exhaustion remains unclear for the time being.