Feeling fatigued after performing intense physical labor is just normal because every physical activity requires continuous muscle and body movement that drains energy. But why do people feel tired after doing work that only involves thinking or brainstorming? A team of researchers from Paris-based Pitié-Salpêtrière University may have found the answer to this question.

Some neurologists argue that mental fatigue is often not real but just a trick our brain plays on us so we pursue things that are less uncomfortable. In contrast, the new study reveals that mental fatigue is not an illusion but a genuine physiological response that involves the accumulation of a toxic substance called glutamate in the prefrontal cortex of the brain. The researchers found that glutamate can affect our decision-making process, proding us to choose tasks that involve less thinking.    

“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, 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,” Mathias Pessiglione, a neuroscientist at Pitié-Salpêtrière University in Paris, said in a statement.

How does glutamate mess with our brain?

If computers can perform tasks like calculations and data processing without a break, why can’t humans? Why do we experience mental fatigue while doing tasks that don’t involve tiresome physical actions? The researchers decided to find answers to such questions by studying two groups of people using a brain imaging technique called magnetic resonance spectroscopy (MRS).  Doctors and scientists often employ MRS to study biochemical changes taking place inside the brain of a living person.

Members of the first group were assigned tasks that required intense cognitive work, while those who were in the second group had to perform comparatively easy mental tasks. While observing both groups for a day, the researchers noticed high glutamate levels in the brains of the members of the first group along with various indicators of fatigue. For instance, there were many people in the first group who experienced pupil constriction and they also engaged in activities provided immediate gratification. 

Meanwhile, no such fatigue patterns were noticed in the members of the second group. The glutamate levels in their brains were low and they didn’t go through any mental burnout. These findings showed that glutamate concentration in the prefrontal cortex of the brain is linked to mental fatigue that arises from intense cognitive work. The researchers suggest that the brain demands a break after or in between a mentally challenging task because, possibly, it needs some time to get rid of the toxic byproduct that accumulates while a person thinks hard.  

So why not get rid of glutamate and become machines?

Glutamate is an important neurotransmitter that plays a critical role in ensuring optimum brain functioning by keeping a check on our mood, memory, and learning ability. This amino acid works as an excitatory neurotransmitter in our central nervous system and therefore, it has the power to stimulate nerve cells so that they readily receive information. Moreover, some studies reveal that glutamate also influences our brain’s ability to adapt in response to different life situations (also called neuroplasticity).

It is when glutamate accumulates in excess that it becomes a problem. The researchers believe that mental fatigue from increased glutamate levels could be actually a defense mechanism to avoid burnout. Glutamate accumulation in the prefrontal cortex isn’t permanent though. The concentration of the chemical becomes normal in the synapses after sleep or some relaxing activity. 

Since mental fatigue also affects our decision-making process, the researchers highlight that a person should not take important decisions when they’re mentally exhausted. When asked, if there are some ways to cancel mental fatigue caused by increased glutamate levels in the brain, Pessiglione said:

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

The researchers are now planning to study the different factors that affect mental fatigue and glutamate concentration in the prefrontal cortex. 

The study is published in the journal Current Biology.