According to the research, a full day of thinking might sometimes feel as tiring as a day of physical labor.

Experts believe they understand why a mentally taxing day can feel as tiring as a day of physical labor.

The accumulation of potentially hazardous by-products in the prefrontal cortex of the brain, according to a recent study, may explain why people feel fatigued after heavy mental exertion.

As mental tiredness sets in, this affects a person’s influence over decisions, causing them to gravitate toward acts that involve no effort or waiting.

Fatigue is the brain’s mechanism of shutting down to preserve itself.

Mathias Pessiglione of France’s Pitie-Salpetriere University stated, “Influential theories indicated that exhaustion is a sort of illusion produced by the brain to induce us to cease whatever we are doing and switch to a more pleasurable pastime.

“However, our data indicate that cognitive work leads to a real functional change – a buildup of noxious substances”; hence, exhaustion would be a signal to stop working for a different reason: to preserve the integrity of brain functioning.

Over the course of a working day, scientists analyzed brain chemistry in two groups of individuals: those who were required to think intensively and those whose cognitive activities were relatively simple.

People in the first group exhibited indicators of exhaustion, such as constricted pupils, and they opted for options that promised rewards with less work and short delays.

In addition, this group exhibited higher amounts of the neurotransmitter glutamate in the prefrontal cortex, according to the study.

This, they argue, lends credence to the notion that glutamate accumulation makes subsequent activation of this portion of the brain so costly that cognitive control is more difficult after a mentally taxing day of work.