Study explores why thinking hard makes you feel tired

Researchers have discovered fresh evidence to explain why intensive thinking causes people to feel mentally weary rather than asleep.

Their findings, published in Current Biology, reveal that when hard cognitive labour is performed for several hours, potentially hazardous wastes accumulate in the prefrontal cortex of the brain. As cognitive weariness sets in, this modifies your influence over decisions, causing you to gravitate toward low-cost activities that require little effort or waiting, according to the study.

“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,” says Mathias Pessiglione of Pitie-Salpetriere University in Paris, France. “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.”

Pessiglione and colleagues including the first author of the study Antonius Wiehler wanted to understand what mental fatigue really is. While machines can compute continuously, the brain can’t. They wanted to find out why. They suspected the reason had to do with the need to recycle potentially toxic substances that arise from neural activity.

To look for evidence of this, they used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday. They looked at two groups of people: those who needed to think hard and those who had relatively easier cognitive tasks.

They saw signs of fatigue, including reduced pupil dilation, only in the group doing hard work. Those in that group also showed in their choices a shift toward options proposing rewards at short delay with little effort. Critically, they also had higher levels of glutamate in synapses of the brain’s prefrontal cortex.

Together with earlier evidence, the authors say it supports the notion that glutamate accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday.

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So, is there some way around this limitation of our brain’s ability to think hard?

“Not really, I’m afraid,” Pessiglione said. “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

Other practical implications may exist. Monitoring prefrontal metabolites, for example, might aid in the detection of severe mental exhaustion, according to the researchers. Such a skill may aid in adjusting work schedules to minimise burnout. He also suggests that individuals avoid making major judgments while they are weary.

They intend to understand more about why the prefrontal brain appears to be more vulnerable to glutamate buildup and tiredness in future investigations. They’re also keen to see if the same tiredness signals in the brain might predict recovery from illnesses like depression or cancer.

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