Thinking Too Hard Really Can Make You Tired, Scientists Say

Youf you’re feeling drained at the end of a demanding day at the office, it could be you’ve been thinking too much.

Researchers compared the chemical makeup of brains from two people in a single day. The one group received simple tasks while the other had to complete more difficult versions of the cognitive assignments. The more complicated tasks had the only sign of fatigue (e.g., reduced pupil dilation).

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Using magnetic resonance spectroscopy, researchers from Pitie-Salpetriere University Hospital in Paris found that high-demand cognitive work 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. The extra makes it difficult to control other activity in the prefrontal cortex area, like decision making and planning. This causes cognitive fatigue, where subjects choose low-effort but high-reward activities.

One of the study’s authors, Mathias Pessiglione, said previous theories suggested fatigue was an illusion concocted by the brain to make us stop what we are doing and turn to a more gratifying 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.”

The monitoring of chemical changes in pre-frontal cortex may have practical consequences, according to researchers. This could help detect mental fatigue and prevent burnout at work.

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