A recent study published in the journal Current Biology shows that prolonged intense cognitive work around 4 to 5 hours causes potentially toxic byproducts to build up in the part of the brain known as the prefrontal cortex, leading to burnout . As cognitive fatigue sets in, behavior begins to change, leading people to seek (albeit unintentionally) ways that require less effort, the researchers explain.
“Influential theories have suggested that fatigue is a type of illusion invented by the brain to get us to stop what we’re doing and turn to more fulfilling activity, but our findings show that cognitive work leads to a leading to real functional change, with an accumulation of harmful substances. So fatigue would indeed be a sign that would make us stop working, but with a different purpose: to preserve the integrity of brain function,” says one of the study’s authors, researcher Mathias Pessiglione from the PitiéSalpêtrière University in Paris., in France.
The researchers used magnetic resonance spectroscopy to monitor brain chemistry throughout a workday. They looked at two groups of people: those who had to do a lot of thinking and those who had lighter cognitive tasks. In the groups that did heavy lifting, they saw signs of fatigue, including decreased pupil dilation, and showed a shift in their choices toward options that offered shortterm rewards with little effort.
This group also had higher levels of the neurotransmitter glutamate at synapses in the brain’s prefrontal cortex. This, along with previous evidence, supports the idea that glutamate buildup makes additional activation of the prefrontal cortex more expensive, so cognitive control is more difficult after a mentally difficult day at work.
For Pessiglione, there is no way around this limitation of our brain’s ability to think a lot. The specialist points to the “good old recipes: rest and sleep”. “There is good evidence that glutamate is cleared from synapses during sleep,” he said.
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