Putting together your taxes is a mental task, but many people are exhausted by the time they are done. The same is true when reading a long report, sorting through a lot of spreadsheet data, or writing a paper with a lot of facts. A new study says that the feeling of being tired after a lot of deep thinking is not all in your head.

Empirical Studies on a tired brain

French researchers wrote in the journal Current Biology on August 11 that lab tests show that work that requires a lot of thinking can cause toxic waste to build up in a part of the brain called the prefrontal cortex.

Researchers say that this changes your ability to make decisions, making you more likely to choose options that are easier or take less time as your brain tires. The prefrontal cortex is the part of the brain where people figure out what to do with different ideas and make important decisions.

Mathias Pessiglione, a senior researcher at Pitié-Salpêtrière University in Paris, said in a written statement, “Influential theories said that fatigue is a kind of illusion that the brain makes up to get us to stop what we’re doing and do something more enjoyable.”

“But our research shows that cognitive work actually changes the way the brain works by causing noxious substances to build up,” he said. “So fatigue would be a signal that tells us to stop working, but for a different reason: to protect the way the brain works.”

Pessiglione and his team put two groups of people to work for a little more than six hours each as part of this experiment.

One group of 24 people was given a difficult mental task. Every 1.6 seconds, letters were shown on a computer, and they had to compare the new letters to ones that had been shown before in different ways. Another group of 16 people was given a similar but simpler task to do.

Researchers used a scanning method called magnetic resonance spectrometry to measure how much glutamate had built up in the prefrontal cortex of the brains of the people who took part in the study.

The Cleveland Clinic says that glutamate is a neurotransmitter that helps people learn and remember. It’s needed in the right amounts and places for your brain to work right, but too much of it can cause neurons to get too excited, which can damage or kill brain cells.

Researchers found that people whose minds had to do harder tasks had more glutamate build-up in the part of their brain called the prefrontal cortex. These people also showed other signs of being tired, such as smaller pupils.

Both groups were also given a financial choice every so often to see if their constant brain work had changed how well they could make decisions. They could choose between a smaller cash reward that was easier to get or a bigger cash reward that took more work or patience to get.

People in the group that was given more difficult brain tasks started to change their economic decisions by the end of the day, choosing more often the easier options that promised rewards quickly with little work.

Dr. Donn Dexter, a fellow of the American Academy of Neurology and neurologist at the Mayo Clinic in Eau Claire, Wis., said that this experiment gives a plausible alternative explanation for brain fatigue.

He said that most experts would say that a tired mind is caused by a lack of energy.

Dexter said, “The brain is continually in need of glucose, although there is a fairly abundant supply of it. This makes little sense. It may be rational to accumulate harmful wastes that hinder performance.”

He said that if that were true, it would explain why sleep, even a short nap, helps the brain feel more alert. There is evidence that our brains get rid of toxins that have built up during the day while we sleep.

But Dexter thought that making decisions about money wasn’t a good way to tell if a brain was tired.

Dexter said, “I would be careful about making the results more general.” “I’d like another study to find the same thing.”

Dr. Thomas Wisniewski, who runs the Center for Cognitive Neurology at NYU Langone in New York City, had the same worries as Dexter about how the study was set up.

Wisniewski said that both the MRS measurement of glutamate levels and the measurements of brain fatigue was “quite indirect.”

“What they are saying makes sense, but all it is is a link. There’s no proof that one thing led to another, “Wisniewski said. “What they’re saying is possible, but this isn’t proof that it’s true.”