Mental pressure can create changes in brain physiology that result in feeling tired. A desire to curl up on the sofa after a day of computer work may be a physiological response to a mentally challenging job, according to a study that ties mental exhaustion to changes in brain metabolism.

The study, published on August 11 in Current Biology1, discovered that participants who worked on intellectually difficult task for more than six hours had greater amounts of glutamate – a key signaling molecule in the brain that could relate to feeling tired.

Too much glutamate can affect brain function, and a rest interval may help the brain to recover correct molecular balance, according to scientists. At the end of the day, these study participants were also more inclined than those who had completed easier activities to choose short-term, easily acquired financial benefits of smaller value over larger prizes that require a longer wait or more effort.

According to Carmen Sandi, a behavioral neuroscientist at the Swiss Federal Institute of Technology in Lausanne, the work is significant in its effort to correlate cognitive weariness with neurometabolic. More research, possibly in non-human animals, is needed, she adds, to demonstrate a causal link between sensations of weariness and metabolic alterations in the brain. “It’s very good to start looking into this aspect,” says Sandi. “But for now this is an observation, which is a correlation.”

Tired brain

Tired brain from hardworking.

A previous study has shown that mental pressure influences physiological indicators such as heart rate variability and blood flow, but the effects are minor, according to Martin Hagger, a health psychologist at the University of California, Merced. 

Cognitive researcher Antonius Wiehler of the Paris Brain Institute and his colleagues hypothesized that cognitive weariness or being tired was caused by metabolic changes in the brain. The researchers enlisted 40 volunteers and allocated 24 to a difficult task and the remaining 16 to an easy task. Both groups worked for somewhat more than six hours, with two ten-minute breaks.

While the study participants concentrated on their job, Wiehler and his colleagues measured glutamate levels in the lateral prefrontal cortex using a technique known as magnetic resonance spectroscopy.

The prefrontal cortex houses cognitive control, which is the region of the brain that permits humans to restrain their urges. “If you get stung by an insect, you want to scratch,” says Wiehler. “If you’re stopping this reflex, that would be cognitive control.” It is also the system on which humans rely when deciding between attractive short-term incentives, such as an unhealthy snack, and long-term gains.

The researchers discovered that those who worked on the more difficult job had more glutamate in this region of the brain at the end of the day than those who worked on the easy assignment. And, when given the option of an immediate financial reward or a larger reward that would arrive months later, they were more likely to choose the smaller, shorter-term reward than they were at the start of the day.

“It would be great to find out more about how glutamate levels are restored,” he says. “Does sleep help? How long do breaks need to be to have a positive effect?” Cognitive fatigue research could also help us understand how professionals react to — and recover from — high-stakes mental labor like air traffic control, where even a brief loss of focus can cost lives.

The discovery of a way to measure glutamate levels in the brain could help scientists explore how the molecule accumulates during mental work and how that affects brain activity the triggers feeling tired , Sandi Hagger says. “Means to detect this have hitherto not been sensitive enough, so this paves the way for future researchers to explore cognitive fatigue”.

Wiehler, A., Branzoli, F., Adanyeguh, I., Mochel, F., & Pessiglione, M. (2022, August 11). A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions: Current Biology. Current Biology; www.cell.com.

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