Feeling Mentally Exhausted? Thinking Too Hard Can Overwork Your Brain, Study Reveals

Researchers reveal the exact reason why overthinking leads to the feeling of brain fog and what you can do about it.

Wallace D. Wattles, an American author from the 19th century, reportedly remarked that “Thinking is the toughest and most demanding of all labor.”

That can seem like a debatable analogy at first. But a recent study reported in Current Biology reveals that, like exercise, thinking too intensely or for too long might exhaust the brain.

Although doing strenuous physical activity is obviously taxing, a person’s sweat or shaking muscles do not indicate how intensely they may be thinking.


As a result, researchers are still puzzled as to why intense contemplating lead to cognitive fatigue. It’s not a feeling of sleepiness or physical tiredness. Rather, it’s the impression that tasks are becoming more difficult to focus on and diminishing willingness to complete them.

We just have to take people at their word when they claim they’re mentally worn out. According to the research, it is due to the buildup of chemical substances that must be eliminated.


The brain’s most prevalent excitatory neurotransmitter, according to some studies, may be responsible for this lack of mental stamina.


Despite being present in more than 90 percent of neuron-to-neuron communications in the human brain, glutamate, an excitatory amino acid, was only described correctly in the 1950s.

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Glutamate is an exceptionally fast-acting neurotransmitter. It contributes to the strength of this amino acid. However, it also makes it difficult to measure the chemical.

This overlooked chemical has kept researchers on their toes for decades. For instance, it has been discovered that neurons regulate the quantity of glutamate they release to other neurons to modulate the strength of their messages in the brain.


Glutamate can excite neurons to death even at the synapse, where up to 8,000 glutamate molecules are crammed into a single pouch. The Synapse is the meeting place of two neurons.

The overproduction of glutamate is undeniably a problem, and this explains why it’s been linked to brain drain.

The research conducted

Twenty-four participants were used for the study. After they have been subjected to demanding computer-based sorting activities for more than six hours, researchers detected an increase in glutamate in their lateral prefrontal cortex. This brain region is connected to higher-order cognitive abilities, such as short-term memory and judgment.

Prefrontal cortex left lateral view
Prefrontal cortex by DBCLS under CC BY-SA 2.1 JP

Comparatively, 16 more people who were given simpler tasks to complete for the day didn’t exhibit any symptoms of glutamate buildup in this region of the brain.

The researchers postulate that one of the factors limiting human mental endurance may be an increase in extracellular glutamate.

The brain naturally consumes a lot of glucose when overthinking.  Other theories speculate that this energy supply may also be a constraint. But biochemically speaking, it is yet unclear how a reduction in glucose complicates the thinking process.


Some scientists have proposed that a drop in glucose causes the brain to lose dopamine, which makes people lose interest in some cognitive tasks, especially those that require arduous thinking more quickly.

Mathias Pessiglione’s opinion

Mathias Pessiglione, a clinical psychologist at the Pitié-Salpêtrière University in Paris, notes that “influential theories indicate that exhaustion is a sort of illusion the brain builds up to make us quit whatever we are doing and move to a more rewarding activity.”

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“Our report shows that the buildup of hazardous substances is a genuine functional alteration brought on by mental exertion. Thus, feeling tired would undoubtedly be an indication that we should stop working, but for a different reason: to maintain the integrity of brain functioning.”


Pessiglione also explained there is good proof that glutamate is excreted from synapses while you sleep. That might contribute in part to the ability to feel mentally rejuvenated the following day after getting a good night’s sleep.

A 2016 functional MRI (fMRI)  study on brain imaging discovered the lateral prefrontal cortex (lPFC) was engaged in high cognitive exertion that gradually decreased its excitability.

It would take considerably more work to activate this region at the end of a long, arduous day or overthinking about that argument you had with your loved one before leaving to work. Hence, the feelings of brain drain.



Pessiglione and colleagues concluded that when these reports were taken together with previous fMRI data, they support a neuro-metabolic model in which glutamate accumulation sets off a regulation mechanism that makes lPFC activation more expensive. It explains why cognitive control is more difficult to mobilize after a demanding workday or to overthink a situation.

Studies like this present one use of the new technology in investigating the role glutamate plays in our brains. The authors are currently looking into why the prefrontal cortex collects glutamate more than other regions of the brain.

Featured image credit: Young man thinking too hard by Shutterstock.com


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