Why does intense thinking make us tired?

While the effects of physical labor on one’s body are well known, scientists are still deciphering the toll mental labor has on us. A new study has now explained why prolonged intense thinking makes us so tired.

Published in the journal Current Biology on Thursday, new research by the France-based Pitié-Salpêtrière University has found that “intense cognitive work” over several hours can lead to “potentially toxic byproducts” to develop in the brain’s prefrontal cortex (PFC).

The PFC is a key area for cognitive control processes, and dopamine in the PFC can affect attention, impulse control, prospective memory, and cognitive flexibility, according to the Textbook of Natural Medicine.

Thus, when the PFC is negatively impacted, it can “alter your control over decisions, so you shift toward low-cost actions requiring no effort or waiting as cognitive fatigue sets in,” the researchers said in a release.

The goal of the study was to understand why, unlike computers, brains can’t compute continuously and face fatigue. The initial hypothesis put forward was that brains need to recycle potentially harmful compounds produced as a result of prolonged brain activity.

“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, one of the study’s co-authors, in a release.

“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 researchers tracked the chemistry of the brain over the course of a workday using magnetic resonance spectroscopy (MRS). They studied two groups of people: those who had challenging cognitive tasks to complete and those who had relatively simple cognitive tasks.

Only in the group engaged in heavy mental labor did they notice indicators of weariness, such as decreased pupillary dilation.

Additionally, those in that group showed a change in their preferences toward alternatives that promised benefits with minimal work and a short wait time.

The study’s authors also point out that the harder-working group was found with larger glutamate concentrations in the PFC of the brain’s synapses, which can make it harder to activate the cortex. This proves their theory that it is difficult to maintain cognitive control after a day of mental labour.

Pessiglione says that while one can’t get around the brain’s limited ability for prolonged thinking, it can be managed.

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

He also advised against making significant decisions when sleep deprived.