Your body may be pushing you to make worse choices after a day of hard thinking, study finds

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(CNN)— After a long day at work, it feels so much better to just leave the dirty dishes in the sink or put off looking at your finances — right?

Don't just call it laziness: After a long period of thinking hard, making decisions that favor ease in the short term but are worse overall appears to be a biological regulation tool to combat cognitive fatigue, according to a new study published Thursday in the journal Current Biology.

"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," said study author Mathias Pessiglione, Inserm research director at the Brain and Spine Institute in Paris, in a news release. "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."

In the study, 40 people were given either an easy or hard version of a task that involved differentiating letters on a screen for more than six hours. The participants reported their levels of fatigue, and researchers used magnetic resonance spectroscopy (MRS) to monitor their metabolic response throughout the study period, according to the study.

Each participant was then offered choices of either an immediately gratifying smaller reward that required less cognitive control or one that was higher value in the longer term but involved some impulse control (for example, I'll give you $10 now or transfer $50 into your bank account tomorrow).

Participants who had to think harder for the six-hour task were more likely to take the smaller reward, according to the study. The researchers found that the harder the participants thought, the higher their levels of glutamate, a neurotransmitter that works in memory and learning.

The results suggested that after people spend long periods of time thinking hard, the glutamate accumulation triggers a response in the brain, making it more difficult to use the prefrontal cortex (the area in the brain that allows us to control our thoughts) so that we make choices that are more impulsive than strategic, the study said. With less controlled thought put into the choices after a long day, the less likely glutamate will continue to accumulate to potentially toxic levels.

If you are about to make an important decision or trying to keep the chores from piling up, it's important to make sure you aren't too tired, said study author Antonius Wiehler, a cognitive neuroscientist and postdoctoral researcher at the Paris Brain Institute.

But bad news: It may also be difficult for people to accurately gauge how fatigued they actually are, according to the study.

Take breaks and try new things

To learn how to beat cognitive fatigue, we first need to recognize when it happens.

You are less likely to become cognitively fatigued by an activity you enjoy than one you don't, said Phillip Ackerman, a professor of psychology at the Georgia Institute of Technology. Ackerman was not involved in the study.

Think of how much more mentally exhausted you may feel after 30 minutes reading a textbook than you would if you stayed up into the wee hours of the night reading a novel, he added.

That said, if you do anything that takes brain power for long enough, you will likely get fatigued, according to Ackerman.

Sometimes there is no avoiding the long stretches of hard thinking, and you have to perform to the best to your abilities. In those cases, how you approach cognitive fatigue can make all the difference, Ackerman said.

"Feeling fatigued is not the same thing as having a performance decrement," he said.

There are three responses people tend to take toward the exhausted feeling: Continue the activity with less effort, focusing to work through the strain, or pushing to think even harder.

The first option often correlates with a dip in performance as the task is given less attention and effort without a rest period to truly recover, he said. The third can be helpful to your thinking and concentration, but if you have to keep going for a long time you risk crashing hard. The second often maintains a similar or even higher level of performance across the concentrated thinking timeline, he added.

In the best-case scenario, people can avoid cognitive fatigue by building in breaks during the difficult thinking, Ackerman said.

Those breaks can be restful for a tired brain if they involve doing a different activity. Even if it involves something else that requires effort, changing things up can help to rejuvenate a tired mind, he said.

That means it could be helpful to break up a long day of intense research with a card game with a friend or a walk outside. And taking the time away can mean that when you do get back to work, what you get out of it is even better.

And true rest helps as well, Pessiglione said.

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

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