Why Thinking Too Much Is Exhausting? Mental Fatigue Due to Toxic Buildup of Chemical in Brain, Study Claims

Researchers have recently shown evidence that explains why hard mental labor is making a person feel very tired. The reason one is feeling mentally exhausted, as opposed to drowsy, from intense thinking is not all in the head.

As indicated in a EurekAlert! report, it is no surprise that "hard physical labor' wears an individual out, hard physical labor is something else. However, sitting around doing hard thinking for hours makes a person feel worn out too.

The new research shows that when intense cognitive work is done for a long period, say, for several hours, it's causing potentially toxic byproducts to build up in the portion of the brain called the prefrontal cortex.

This in turn changes the control over decisions, and thus, one shift toward low-cost actions requiring no effort or waiting as cognitive fatigue sets in explained the researchers.

Mental Fatigue

According to Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France, influential theories suggested that fatigue is a "sort of illusion" cooked up by the brain to make a person stop whatever he is doing and turn to a more gratifying activity.

However, Passiglione also said, their findings demonstrate that cognitive work leads to a true functional change, and accumulation of noxious substances, so fatigue would definitely be an indication that makes one stop working although for a different purpose and that is to preserve the integrity of brain functioning.

Pessiglione, together with colleagues including Antonius Wiehler wanted to know what mental fatigue really is. 

According to them, while machines can continuously compute, the brain cannot do so. In their research published in Current Biology, they wanted to discover the reason why thinking too much leaves one exhausted.

Glutamate Accumulation

The team suspected that the reason had to do with the necessity of recycling potentially toxic substances that occur from neutral activity.

To look for evidence, they used magnetic resonance spectroscopy or MRS to monitor brain chemistry over the period of a workday.

They looked at two groups of individuals - those who needed hard thinking and those who had relatively easier tasks related to cognition.

As a result of their search, the study investigators saw indications of fatigue, which include reduced pupil dilation, only in the group that did the work.

Those in the said group also demonstrated in their choices a switch toward options that propose a brief delay with little effort. Critically, they had higher glutamate levels in synapses of the prefrontal cortex of the brain. 

Along with previous evidence, the researchers said it backs the notion that glutamate accumulation makes further activation of the prefrontal cortex more expensive such that cognitive control is more challenging after a mentally tough workday