Study: Thinking hard makes people tired because of ‘noxious’ accumulations in brain

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Study: Thinking hard makes people tired because of 'noxious' accumulations in brain

New scientific evidence released Thursday suggests why thinking hard makes a person fatigued: noxious substances building up in the brain. Photo by Tigger11th/Shutterstock

Even an “up and at ’em” sort of person may feel exhausted and need to rest after taking a big exam as much as after running a race. New scientific evidence released Thursday may help explain why thinking hard makes a person tired, beyond drowsiness: noxious substances building up in the brain.

Specifically, intense cognitive work over several hours was found to result in glutamate buildup in the brain’s lateral prefrontal cortex. And this reduces a person’s control over decisions so they shift toward “effortless behaviors with immediate gratifications.”

That’s the gist of research findings published in Current Biology.

The new scientific paper notes that, despite more than a century of research, the origins of cognitive fatigue have remained elusive.

“Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity,” study co-author Mathias Pessiglione said in a news release.

However, he said, the new study’s findings “show that cognitive work results in a true functional alteration — accumulation of noxious substances — so fatigue indeed would be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

Pessiglione works as a researcher in the Motivation, Brain & Behavior lab at Pitié-Salpêtrière University in Paris.

Glutamate is described by the Cleveland Clinic as “the most abundant excitatory neurotransmitter” in the brain and central nervous system, and is needed to maintain proper brain functioning.

But experts say glutamate must be present in the right concentrations in the right places. Too much glutamate in the brain may be toxic.

For their study, the French investigators sought to determine why machines can compute continuously but the brain cannot, the release said. They suspected the reason was associated with the need to recycle potentially toxic substances arising from neural activity.

Seeking evidence of this, the scientists used magnetic resonance spectroscopy to monitor brain chemistry over the course of a work day.

They analyzed two groups: people who needed to think hard and those who had relatively easier cognitive tasks.

The researchers saw signs of fatigue, including reduced pupil dilation, only in the group doing hard work. When making choices, the hard-work group also shifted toward options proposing rewards after only short delay and with minimal effort. And they had higher levels of glutamate in synapses of the brain’s prefrontal cortex.

Together with earlier evidence, the investigators say their research supports the idea that glutamate accumulation in the brain makes cognitive control more difficult after a mentally tough work day.

Pessiglione said there’s no way around this self-limiting aspect of hard thinking — except rest and sleep.

“There is good evidence that glutamate is eliminated from synapses during sleep,” he said.

The researchers added that how this cognitive fatigue relates to other forms of fatigue remains unclear.

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