It’s no surprise that physical labor is tiring, but what about mental labor? Sitting and thinking for hours on end also exhausts you, and scientists explain why.
A busy day at work with a series of mentally demanding tasks can leave a person feeling drained, and they are more likely to choose a relaxing evening in front of the television rather than tackling a tough new task afterwards. Moreover, the probability of preferring a smaller but immediate reward is much greater than the remote chance of a better reward.
When there is intense and prolonged cognitive work for several hours, potentially toxic chemicals are produced and accumulate in the part of the brain known as the prefrontal cortex. This, in turn, disrupts brain function and impairs control over decisionsthat leads the person to decide for actions that do not require more effort, explain the scientists in an article published in the journal Current biology.
“There are theories that suggest that fatigue is a kind of illusion invented by the brain to make us stop what we are doing and turn to a more fulfilling activity”, says Mathias Pessiglione of the Pitié-Salpêtrière University in Paris.
Our research shows that intense cognitive work causes a real functional change – an accumulation of harmful substances. Thus, fatigue is indeed a signal to make us stop working, but with a different objective: to preserve the integrity of the functioning of the brain.
What is mental fatigue?
The team of Mathias Pessiglione and Antonius Wiehler from Pitié-Salpêtrière University wanted to understand how mental fatigue is determined.
To figure out what’s going on in the brain, they used magnetic resonance spectroscopy to monitor brain chemistry over the course of a working day. They analyzed 40 volunteers, divided into two groups: those who had a task they had to think a lot about and those who had relatively easier cognitive tasks.
- They only saw signs of fatigue, including reduced pupil dilation, in the group that had to think a lot.
- Their choices shifted to options that offered short-term rewards with little effort.
- They had higher levels of the neurotransmitter glutamate at synapses in the brain’s prefrontal cortex.
Combining the data with previous research, the scientists claim that the “The accumulation of glutamate makes it increasingly difficult to activate the prefrontal cortex, so cognitive control is more difficult after a mentally intense day’s work.”
Scientists believe that mental fatigue occurs so that the brain shuts down and is able to recycle potentially toxic substances that result from neural activity.
The brain is not like a computer that runs all the time.
Is there a way around this limitation in our brain’s ability to keep functioning?
“Unfortunately, no,” Pessiglione said. “I recommend the good old recipes: rest and sleep. It is proven that glutamate is eliminated from the synapses during sleep”
The scientist also advises people to avoid making important decisions when they are tired.
This study may have other practical implications, for example, monitoring glutamate accumulation may help detect severe mental fatigue in order to adjust work schedules and prevent burnout.
Another area of interest is research into diseases of which fatigue is a symptom, such as chronic fatigue syndrome. The presence of glutamate may explain why patients feel so exhausted.
But there are still many other open questions. Scientists want to conduct more studies to find out why the prefrontal cortex seems particularly sensitive to glutamate buildup. They’re also curious if the same fatigue markers in the brain can aid recovery from diseases such as depression or cancer.