Here’s why you’re so tired after a long day at your desk

A long day of deep thought can be as exhausting as a day of manual labor, and scientists say they’ve discovered why.

The researchers suggest that their findings show why people feel mentally tired rather than sleepy.

Studies show that after hours of intense mental work, toxic byproducts build up in a part of the brain known as the prefrontal cortex.

This changes human control over decision making, switching to effortless actions or waiting when mental fatigue sets in.

This fatigue tells the brain to shut down to save itself.

“The prevailing theory is that fatigue is an illusion designed by the brain to get us to stop what we’re doing and move on to more pleasurable activities,” said Matthias Pesiglione of France’s Pitiers-Salpetriere University.

“However, our findings show that cognitive work leads to actual functional changes, the accumulation of toxic substances. So while fatigue is certainly a signal to stop working, it has another purpose: to maintain the integrity of brain function.” .

The researchers looked at brain chemistry throughout the day, looking at two groups of people who needed to think a lot and those who performed relatively simple cognitive tasks.

Signs of fatigue, including decreased pupillary dilation, were seen in the heavy work group.

This group of people also chose options that offered short delays with little effort.

Importantly, the researchers also noted higher levels of the chemical glutamate in the brain’s prefrontal cortex.

Combined with previous evidence, the authors say they support the idea that glutamate accumulation is costly for further activation of this part of the brain.

The findings are published in the journal Current Biology.