Have you ever felt exhausted at the end of a day when all you did was sit at a desk? It could be that you thought too hard.

In a recent study, researchers from Pitie-Salpetriere University Hospital in Paris organized two groups of people. One group was given easy cognitive assignments, while the other group carried out more demanding versions of the same tasks. The scientists then analyzed the brain chemical composition of the two groups of people over an approximate work day. They found signs of fatigue, such as reduced pupil dilation, only in the group that was given the harder tasks.

More specifically, the scientists found that high-demand learning led to a build-up of glutamate—a chemical that nerve cells use to transmit signals to other cells—in the pre-frontal cortex area of the brain by using magnetic resonance spectroscopy.

The practical result is dealing with this excess of glutamate makes other pre-frontal cortex activity, like planning and decision making, more difficult, leading people to prefer low-effort, high-reward actions as fatigue sets in.

Mathias Pessiglione, one of the study’s authors, said previous theories suggested fatigue was a brain trick to make us seek a more satisfying activity. “But our findings show that cognitive work results in a true functional alteration—accumulation of noxious substances—so fatigue would indeed be a signal that makes us stop working, but for a different purpose: to preserve the integrity of brain functioning.”

Perhaps monitoring chemical changes in the brain’s pre-frontal cortex could help detect mental fatigue to avoid burnout in the workplace? Now it seems quite possible.

Taken from “Thinking Too Hard Really Can Make You Tired, Scientists Say” by Alex Millson (https://time.com/6205432/mental-fatigue-no-energy/?)