Making Big Decisions After A Difficult Workday Is Hard, According to a New Study It's not just you -- it's a chemical in your brain called glutamate.

Using your brain all day literally wears you out — and makes it harder to make bigger decisions, according to a new study.

According to the study, cognitive fatigue has been "conceived as an inflated cost of cognitive control, objectified by more impulsive decisions," in which people lose the ability to control their thought processes as easily and make more spur-of-the-moment decisions.

Scientists had various explanations for why people experience cognitive fatigue, according to the study, which was published last week in the Current Biology journal.

One explanation is that the brain does it on purpose, "to make us stop whatever we are doing and turn to a more gratifying activity," says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France, said in a press release.

It's not an illusion generated by the brain, this study posits.

"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," Pessiglione said in the statement.

Why is it harder to make decisions after a long day?

The study found that after several hours of performing difficult tasks, people had more glutamate in the lateral prefrontal cortex, which is responsible for judgment and solving problems.

Having glutamate present in the correct amounts at the optimal times is associated with better brain function, but having too much has been linked to brain diseases, according to Cleveland Clinic.

When the chemical piles up — maybe like emails at the end of a long day — it's harder for your brain to work, according to the release.

"While machines can compute continuously, the brain can't," the release added.

There were a few other indicators of cognitive fatigue. In the study, the group that performed difficult tasks all day tended to choose "lower-cost" decisions later, like choosing immediate, shorter-term rewards over harder-to-make but more lucrative decisions.

The trick, according to the release, is to not even to ask yourself how tired you are — the subjective reporting measure results showed a discrepancy between how tired people reported feeling and other cognitive function measures — but to lean on an old-fashioned cure: rest.

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

One other way? Do something you enjoy. Phillip Ackerman, a professor of psychology at the Georgia Institute of Technology who was not involved in the study, told CNN that you are less likely to become cognitively fatigued by an activity you enjoy than the one you don't.