(CNN) — After a long day at work, it feels so much better to leave the dirty dishes in the sink or review your finances for later, right?
This is not just laziness. After a long period of thinking hard, making decisions that favor short-term ease but are worse overall appears to be a biological regulatory tool to combat cognitive fatigue, according to a new study published Thursday in the academic journal Current Biology.
“Influential theories suggest that fatigue is a kind of illusion cooked up by the brain so that we stop what we’re doing and engage in more rewarding activity,” said study author Mathias Pessiglione, director of Inserm research at the Institute of Brain and Spine of Paris, in a press release. “Our findings show that cognitive work causes a true functional alteration, an accumulation of harmful substances, so fatigue would indeed be a signal that makes us stop working, but with a different purpose: to preserve the integrity of brain function.”
In the study, 40 people were given 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 the researchers used magnetic resonance spectroscopy (MRS) to monitor their metabolic response throughout the study period, according to the study.
Each participant was then offered the choice between a smaller, more immediately rewarding reward requiring less cognitive control, or one of greater long-term value but involving some impulse control (for example, I’ll give you $10 now or I’ll transfer $50 to your bank account tomorrow).
According to the study, participants who had to think the most during the six hours of the task were more likely to accept the smaller reward. The researchers found that the more the participants thought, the higher their levels of glutamate, a neurotransmitter involved in memory and learning.
The results suggest that after people spend long periods of time thinking intensely, the buildup of glutamate triggers a response in the brain that makes it harder to use the prefrontal cortex (the area of the brain that allows us to control our thoughts) so that we make more impulsive decisions than strategic ones, according to the study. The less controlled thought you put into your choices after a long day, the less likely it is that glutamate will continue to build up to potentially toxic levels.
If you’re about to make an important decision or trying to keep tasks from piling up, it’s important not to be too tired, said study author Antonius Wiehler, a cognitive neuroscientist and postdoctoral researcher at the Paris Brain Institute.
But the bad news is that, according to the study, it can also be difficult to accurately measure the degree of fatigue.
Take breaks and try new things
To learn how to beat cognitive fatigue, you must first recognize when it occurs.
An activity you like is less likely to fatigue you cognitively than one you don’t, says Phillip Ackerman, a psychology professor at Georgia Institute of Technology. Ackerman was not involved in the study.
Think about how mentally exhausted you can feel after 30 minutes reading an academic book than if you stayed up late at night reading a novel, he added.
That said, according to Ackerman, if you do anything that requires brain power for a long time, you’re likely to become fatigued.
Sometimes you can’t avoid long periods of hard thinking, and you have to perform at your best. In those cases, how you deal with cognitive fatigue can make a difference, says Ackerman.
“Feeling fatigued is not the same as having decreased performance,” he said.
There are three responses people tend to adopt when feeling exhausted: continue the activity with less effort, concentrate to work through the stress, or try even harder.
The first option is often correlated with decreased performance, as less attention and effort is put into the task without a period of rest to truly recover, he said. The third can be useful for thinking and concentration, but if you have to go on for too long you run the risk of going downhill.
The second tends to maintain a similar or even higher level of performance throughout the time spent in concentrated thought, he added.
At best, people can stave off cognitive fatigue by pausing during difficult thinking, Ackerman said.
Those breaks can be restorative for a tired brain if they involve doing a different activity. Even if it’s something that takes effort, changing things up can help rejuvenate a tired mind, she noted.
That means it can be helpful to break up a long day of intense research with a game of cards with a friend or a walk in the fresh air. And taking that time can mean that when you get back to work, you put in an even better effort.
And true rest helps, too, says Pessiglione.
“I would use the old recipes: rest and sleep! There is strong evidence that glutamate is cleared from synapses during sleep,” he said in the statement.