Mental fatigue? It's all in the brain, says the science

File image.

File image.

Published Aug 13, 2022


Johannesburg - Thinking hard makes your brain hurt and finally there is the science to prove it.

For a long time those who complained that they were feeling tired after stretches of power thinking were told that it was all in their heads.

But now scientists have pinpointed just why you feel mentally exhausted after a night of cramming for an exam or sitting through a day long meeting.

It has all got to do with toxins.

In a study released on Thursday in the journal Current Biology, researchers found that an intensive cognitive work out causes potentially toxic by-products to build up in the part of the brain called the prefrontal cortex. When this happens the brain, the researchers explained, gears down and shifts to low cost actions that require less effort and with it comes that tired feeling.

“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,” said Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France, in a statement.

“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.”

Pessiglione and his colleagues wanted to unpack what mental fatigue was, although they already had a hunch that it might be the brain having to recycle toxins that had accumulated because of extra neural activity.

To get a peek into what was happening in the brain during these cognitive workouts, they used magnetic resonance spectroscopy.

Two groups of volunteers were gathered for the experiment, which was conducted over a working day.

The first group were required to complete tasks where they had to think hard, while the second lot of volunteers took it easy and handled far simpler cognitive tasks.

What the researchers found in the hard working group were tell tale signs of fatigue that included reduced pupil dilation.

This group also began choosing tasks with little effort. They were also found to have had higher levels of glutamate in the synapses of their brain’s prefrontal cortex.

The authors were able to conclude that the accumulation of glutamate made further activation of the prefrontal cortex more costly, and this in turn made cognitive control difficult after a long day of heavy mental lifting.

The research could one day help in preventing burnout. Through the monitoring of prefrontal metabolites medical professionals might be able to detect severe mental fatigue and adjust work loads accordingly. It might have applications in predicting recovery from health conditions, such as depression or cancer.

Pessiglione also advised people to avoid making important decisions when they’re tired.

Unfortunately energy drinks and caffeine won’t help in giving the prefrontal cortex a pick me up. It comes down to a bit of age old advice.

“I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep,” Pessiglione said.

The Saturday Star