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Why Does Thinking Hard Make Us Tired?

My Mind News examines why thinking hard can cause additional fatigue to what we routinely experience as part of our everyday lives.

Why is mental labor tiring?

We are all well aware that hard physical labor can wear us out. But we must also consider mental labor, too. Spending significant time thinking hard, perhaps for hours, can also make us feel fatigued and maybe even worn out., or mentally exhausted.

Researchers have recently reported new evidence to explain why this is, and, based on their findings, the reason you feel mentally exhausted (as opposed to simply drowsy) from intense thinking.

Their studies, reported in Current Biology in August 2022, show that when intense cognitive (mental) work is prolonged for several hours, it causes potentially toxic byproducts to build up in the part of the brain known as the prefrontal cortex.

This process, in turn, alters your control over decisions. As a result, the research reports that you shift toward low-cost actions requiring no effort as cognitive fatigue sets in.

According to Mathias Pessiglione, part of the study team based at Pitié-Salpêtrière University in Paris, France,

“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.”


What is mental fatigue?

Pessiglione and his research colleagues wanted to understand what mental fatigue is. While machines can compute continuously, the human brain can’t. They tried to find out why.

The team suspected this might be due to the need to recycle potentially toxic substances that arise from neural activity. To look for evidence of this, they used a process known as magnetic resonance spectroscopy (MRS) to monitor brain chemistry throughout a workday.

They examined two groups of people – those who needed to think hard and those who were given relatively more straightforward cognitive tasks.

The research team noticed signs of fatigue, including reduced pupil dilation, only in the group doing hard work. The group members also showed in their choices a shift toward options proposing rewards at short delay with little effort. Critically, they also had higher levels of glutamate in the synapses of the brain’s prefrontal cortex.

Synapses are the microscopic gaps that separate one neuron’s terminal buttons from another neuron’s receptors. When neurons communicate, they release chemicals that must travel across this gap to stimulate the post-synaptic receptors. If this gap is somehow blocked or compromised, the brain’s processing power is reduced, and fatigue can result.

Together with earlier evidence, the authors say it supports the notion that glutamate accumulation makes further activation of the prefrontal cortex more costly from an energy perspective, such that cognitive control is more difficult after a mentally tough workday. With reduced energy levels comes tiredness, malaise, and extreme fatigue.

Can we influence the brain’s ability to think hard?

So, given the results of the research, we might be curious as to whether there is some way around this limitation of our brain’s ability to think hard. In response to such a question, Mathias Pessiglione responds,

“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.”

There may be other practical implications. For example, the researchers say monitoring brain toxin levels could help to detect severe mental fatigue. Such an ability may help adjust work agendas to avoid burnout among employees. Pessiglione also advises people to avoid making important decisions when they’re tired.

What next for the research team?

In future studies, the team from Pitié-Salpêtrière University hopes to learn why the prefrontal cortex part of the brain seems especially susceptible to glutamate accumulation and resultant fatigue.

They are also curious to learn whether the same markers of fatigue in the brain may predict recovery from health conditions such as depression or cancer. As the team progresses with its research, My Mind News will report on any new findings that shine further light on this fascinating subject. Just don’t think about it too hard..!


What do you think about the research team’s findings? Do you feel more tired at nighttime when you have been concentrating hard all day? Let us know in the comments. 


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