Why does thinking make you tired?

Does thinking make you tired or is it all just in your head? Researchers explain why mental labour makes us feel mentally exhausted

Physical labour makes us feel physically tired – but can mental labour have the same effect?

Although mental labour does not require physical movement, a study published in Current Biology shows that thinking hard can make us feel worn out. Researchers explain why this is the case.

The research team claim that when intense cognitive work is prolonged for several hours, it causes potentially toxic byproducts to build up in the part of the brain known as the prefrontal cortex.

In turn, this cognitive fatigue affects your control over decision-making and pushes us towards low-cost actions requiring little or no effort or waiting.

Fatigue is not an allusion

“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,” explains Mathias Pessiglione of Pitié-Salpêtrière 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.”

How does thinking make you tired?

The aim of the team was to understand mental fatigue. Machines are able to compute continuously in a way that the brain cannot and Pessiglione and his colleagues wanted to know why.

The researchers suspected that this had something to do with the brain’s need to recycle potentially toxic substances that arise from neural activity.

They began looking for evidence to support this theory and used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday. By analysing two groups of people, those who needed to think hard and those who had relatively easier cognitive tasks, they were able to collect enough data.

The group who had more thought-provoking work suffered from signs of fatigue, including reduced pupil dilation. Those who were tasked with relatively easy cognitive jobs were not fatigued in the same way.

The group who had to think hard also shifted towards rewards at a short delay with little effort. Critically, they also had higher levels of glutamate in synapses of the brain’s prefrontal cortex.

Alongside other evidence, the authors are convinced that glutamate accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday. The evidence proves that thinking really does make us tired!

How can we combat brain fatigue?

Employ good old recipes: rest and sleep!

Pessiglione explains that there is no simple solution to combatting mental fatigue but adds: “I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep.”

However, researchers have said that monitoring prefrontal metabolites may detect severe mental fatigue and avoid burnout.

Pessiglione implores that people do not make important decisions when tired because fatigue affects the way we think.