Why scientists are saying too much thinking is bad for you

The link between physical labour and tiredness is well established, but now researchers have a theory as to why cognitive activity leads to fatigue

When intense cognitive work is prolonged for several hours, it causes potentially toxic by-products to build up in the brain, a study has found. Getty Images

Advice not to overthink things has been given scientific credulity by research published on Thursday which suggests thinking hard adversely affects the brain.

While machines can compute continuously, the brain can’t, and researchers from France wanted to find out why.

Their study showed that when intense cognitive work is prolonged for several hours, it causes potentially toxic by-products to build up in the part of the brain known as the prefrontal cortex. This in turn alters control over decisions, shifting the mindset towards low-cost actions requiring no effort or waiting as cognitive fatigue sets in.

To look for evidence, the researchers used magnetic resonance spectroscopy 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 cognitive work. Those in that group also showed in their choices a shift towards 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 said it supports the notion that glutamate accumulation makes further activation of the prefrontal cortex more costly, with cognitive control more difficult after a mentally tough workday.

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

“But our findings show that cognitive work results in … an accumulation of noxious substance so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning.”

No substitute for sleep

Mr Pessiglione said there is no way round this effect but that it can be mitigated.

“I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

There may be other practical implications of the findings, for example the research suggested monitoring of prefrontal metabolites could help to detect severe mental fatigue, militating against burnout.

In future studies, the researchers hope to learn why the prefrontal cortex seems especially susceptible to glutamate accumulation and fatigue. They’re also curious to learn whether the same markers of fatigue in the brain may predict recovery from health conditions, such as depression or cancer.