A long day of thinking can feel just as tiring as a day of hard labour, and now we know why.

According to a recently-published study in the journal Current Biology, the all-too-common feeling of being mentally worn out is due to build-up of glutamate, a chemical in the brain’s prefrontal cortex that is useful but dangerous in high doses.

An important messenger in our brains

Normally, glutamate acts as a neurotransmitter, or “chemical messenger”, that stimulates the nerve cells in our brains to send messages between them. This helps us learn and process information, with glutamate also allowing for nerve cells to build the foundations of our memories.

But, when we have too much glutamate in our brains the nerve cells can become overexcited, which can lead to the cells becoming damaged or dying. Glutamate levels that are too high are also associated with several conditions that affect the brain, including Alzheimer’s, Parkinson’s, chronic fatigue syndrome, strokes, and multiple sclerosis.

In this new study, the team of researchers say a build-up of glutamate in the prefrontal cortex alters our control over decisions and makes the continued use of our prefrontal cortex more energy-consuming. As a result, we’re more likely to make decisions that are low effort and have high rewards.

Wanting to understand what mental fatigue actually is, Mathias Pessiglione and his colleagues at the Pitié-Salpêtrière University in Paris monitored the brain chemistry of two groups of people over the course of a workday.

One group, who were performing mentally difficult work, showed signs of fatigue such as reduced pupil dilation where the group performing relatively easier tasks didn’t.

The team also noticed that members of this group began to shift their choices towards tasks that were little effort but came with high rewards they didn’t have to wait long for.

Importantly, the team found that the group doing hard work had higher levels of glutamate in the synapses of the prefrontal cortex.

Their findings also go against popular theories that mental fatigue is all in our head.

“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,” Pessiglione said.

“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.”

Can we deal with glutamate build-up?

Though we can now explain why we’re feeling so tired, Pessiglione said there are only two ways to deal with the build-up of glutamate in our brains and our fatigue.

 “I would employ good old recipes: rest and sleep!” he said.

“There is good evidence that glutamate is eliminated from synapses during sleep.”

Even so, the findings from Pessiglione and his team could have practical implications.

For example, the researchers say that monitoring prefrontal metabolites – such as glutamate – could help detect severe mental fatigue, which could be beneficial for managing work agendas to avoid burnout.

Pessiglione also advised people to avoid making important decisions when they’re tired.

In the future, the team hopes to discover why the prefrontal cortex is so susceptible to fatigue and the accumulation of glutamate, and whether these markers for fatigue could be used to predict how people recover from conditions such as cancer or depression.

Image: Getty Images

This article first appeared on OverSixty.