Why Does a Long Day of Thinking Tire You Out?

A long day of writing emails, making phone calls and attending Zoom meetings can make you want to plop on the couch and close your eyes—even if you’ve already spent most of the day sitting in a chair. Mental fatigue is a well-known experience, but scientists aren’t entirely sure why some cognitive tasks feel so draining.

Researchers have long been searching for the cause of this exhaustion. Studies in the early 2000s suggested that mental effort depletes the brain’s supply of energy-providing glucose, but further studies were unable to replicate those findings, per Scientific American’s Diana Kwon.

In a new paper, published last week in Current Biology, researchers suggest the buildup of glutamate, a chemical neurons use to send messages to each other, could be a contributing factor.

“The puzzling thing is why some mental activities are perceived as effortful at all and lead to fatigue, while others are virtually automatic and do not lead to fatigue (like vision),” Alexander Soutschek, who studies decision making and self-control at Ludwig-Maximilians-University of Munich, tells Vice’s Shayla Love. Soutschek did not contribute to the new study.

In the recent experiment, researchers split 40 paid participants into two groups. In one, participants attempted cognitively challenging tasks, such as remembering whether a number matched one they’d seen earlier, while a second group did easier activities, per Science’s Emily Underwood. The experiment lasted around six hours with two ten-minute breaks—a duration meant to approximate a workday.

As the participants worked, the researchers measured their glutamate levels in a part of the brain called the lateral prefrontal cortex, which is used to suppress impulses. In a previous study, the researchers had seen reduced activity in that part of the brain after performing cognitive tasks, per Vice.

This time, researchers found more glutamate buildup among the participants who received the challenging tasks than those who received simpler ones, reports Nature News’  Heidi Ledford. The two groups had no difference in glutamate levels in the visual cortex, which acted as a control, according to Vice.

Performing the cognitive tasks also seemed to have an impact on impulse control. Compared to before the experiment, participants in the challenging group were more likely to turn down a future monetary reward in favor of an immediate, though smaller, cash reward, according to Nature News.

The research doesn’t mean that excessive glutamate specifically causes mental fatigue, notes Vice. Nor does it prove that thinking boosts glutamate levels. “We’re still far from the point where we can say that working hard mentally causes a toxic buildup of glutamate in the brain,” Antonius Wiehler, first author of the new paper and a computational psychiatrist at Le Groupe Hospitalier Universitaire Paris, tells Science.

The authors hypothesize that participants performing challenging tasks made more impulsive choices in part because their brains don’t want glutamate to build up. “The brain is monitoring [glutamate levels], and to avoid the accumulation of glutamate, the brain is reducing its activity, and participants are applying less control during their choices, and therefore their choices change,” Wiehler tells Vice.

Other scientists aren’t convinced glutamate is a primary cause of mental fatigue. Jonathan Cohen, a neuroscientist at Princeton University who wasn’t involved in the research, tells Science that the brain has to have a more sophisticated system for juggling many demanding tasks. “It just can’t be that easy,” he says.

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Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.