Ever get home from the office and feel exhausted, even though you’ve sat at a desk all day? Scientists think they have figured out the reason why thinking hard can make you tired, giving new meaning to a “mental vacation.”
A group of researchers at the Paris Brain Institute have shown that tasks that make you use your brain are associated with an increase in a key neurotransmitter called glutamate. Glutamate buildup doesn’t just muck up your brain, they argue—it affects your ability to make decisions as you become more and more fatigued. The study was published on Thursday in the journal Current Biology.
According to study co-author Mathias Pessiglione, a brain and behavior researcher at the Paris Brain Institute, neuroscientists previously thought that mental fatigue was an illusion created by the brain to encourage a person to switch tasks when the one at hand became boring or repetitive.
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But Pessilione and his team have found that fatigue goes deeper, instead acting as a stop-gap before too many noxious chemicals can build up.
“Fatigue would indeed be a signal that makes us stop working, but for a different purpose: To preserve the integrity of brain functioning,” he said in a press release.
To get to these findings, the researchers split 40 participants up into “easy” and “hard” groups and tasked them with completing brainy tasks for nearly six and a half hours, with just two 10-minute breaks interspersed through the trial. In both groups, they alternated between working through exercises designed to make them tired (distinguishing between vowels vs. consonants or uppercase vs. lowercase letters, depending on the letters’ colors) and ones that gauged how tired they became (economic decisions, like whether to receive 41.20 euros now or 50 euros in a month).
For three stints during this process, the researchers measured these participants’ brain biochemistry, finding that as the session wore on, the people assigned to the hard group accumulated more glutamate in the prefrontal cortex region of their brains. The hard group also started choosing answers to the economic deliberations that required less thinking and calculating, and their pupils dilated less than the easy group’s over time—a measure that connotes fatigue and cognitive effort.
Interestingly, when asked to subjectively rate their level of fatigue, the two groups scored themselves similarly over time. The researchers took this to mean, simply, that people may not be very good at listening to their bodies.
“This dissociation is common in everyday life; for instance, when people go on working or driving and start making errors because they failed to detect their true fatigue state,” the researchers wrote in the study.
And of course, deep thinking can take place in any situation, not just during the workday. Take the example of professional chess players: Even they make mistakes, “typically after 4–5 h in the game that they would not make when well rested,” the authors wrote.
But what does glutamate buildup mean? The neurotransmitter needs to be present at low levels for the brain to function properly, but when it’s overproduced or released in the wrong areas, it could be a toxic by-product, the researchers wrote. Still, further work must show that excess glutamate produces mental fatigue, not just that the two are linked.
And unfortunately, Pessiglione said there’s no easy hack to reversing the potentially toxic effects of a hard day’s work:
“I would employ good old recipes: rest and sleep,” Pessiglione said.
