Mental fatigue: Why thinking hard makes you feel worn out

Stressed man

The reason you feel mentally exhausted from intense thinking isn’t all in your head.

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What you need to know:

  • Scientists have found that many hours of concentration lead to a build-up of a compound known as glutamate that accumulates in the front regions of the brain, leading to mental fatigue.
  • This explains why we avoid difficult tasks when mentally fatigued -  the glutamate overload makes further mental work difficult.  

It’s no surprise that hard physical work wears you out, but what about hard mental labour? Well, sitting around thinking hard for hours makes one feel fatigued, too.

Scientists have new evidence to explain why this is the case, and, based on their findings, the reason you feel mentally exhausted from intense thinking isn’t all in your head.

The scientists have found that many hours of concentration lead to a build-up of a compound known as glutamate that accumulates in the front regions of the brain, leading to mental fatigue.

This explains why we avoid difficult tasks when mentally fatigued -  the glutamate overload makes further mental work difficult.  “Too much glutamate is potentially harmful, the brain wants to avoid this and so it is trying to reduce activity,” Antonius Wiehler, lead researcher at the Paris Brain Institute in France explained while highlighting that though many people have experienced mental weariness after a hard day of thinking, until now, experts didn’t know why.

“The brain doesn’t seem to run out of energy after working hard and even when we aren’t deliberately thinking about anything specific, some brain regions called the ‘ default mode network’ are as active as ever .”

 Wiehler and his team employed a technique called magnetic resonance spectroscopy (MRS), which measures levels of various chemicals in living tissue harmlessly and specifically focused on a region towards the front and sides of the brain called the lateral pre-frontal cortex that much previous work has shown is involved in difficult mental tasks.

The team asked 40 people to do memory tasks while lying in an MRS scanner and these included watching sequences of numbers appear on a screen and stating if the current number was the same as a previous one.

Twenty-six of the participants did a harder version of this task, while the other 14 were given an easier one after which levels of eight different brain chemicals were measured, including glutamate, which is the main signalling chemical between neurons.

After completing the memory tasks for six hours, those doing the harder version had raised levels of glutamate in their lateral pre-frontal cortex while in those doing the easier task, levels stayed about the same.

They observed that across all participants, there was no rise in the other seven brain chemicals that were measured. For the participants doing the harder tasks, their glutamate level rise tallied with dilation of the pupils in their eyes, another broad measure of fatigue.

They then went ahead to investigate if mental fatigue affected decision-making by interspersing the memory task with different exercises, such as one where people choose between getting a sum of money straight away or a different one later.

The team observed that as the participants doing the harder task felt more tired and had an accumulation of glutamate, they shifted to options that gave a small reward immediately and this could be an example of us avoiding difficult mental tasks, such as calculating which choice to make, to prevent the accumulation of potentially harmful glutamate levels.

According to the lead researcher, one way to (reduce glutamate build-up) is to activate the lateral pre-frontal cortex less during choices.

“If you do that, you are more often choosing the tempting option.”

Experts believe that measuring brain glutamate could be used to reveal how hard a region of the brain has been working and their findings could be used to assess people with concentration difficulties.