How Mental Exhaustion Changes the Choices You Make

This scenario will sound familiar to many parents: imagine spending the day watching your kids: making them breakfast, settling arguments, answering their questions, and trying to keep them from getting bored. At the end of the day, you finally take a moment to think about what you’re going to make for dinner and, with that, whether you have the ingredients, how long it will take, and whether your kids will eat them. After an exhausting day, even simple tasks like preparing a meal can seem overwhelming.

This feeling is cognitive fatigue, and it can occur in all aspects of life. This is why an office worker doing menial computer tasks all day may feel too tired to go out with friends after work. And it’s also why a nurse who has just finished a long and demanding shift might choose not to visit an additional patient despite her initial intention to check on someone.

In search for our team At Johns Hopkins University and the Kennedy Krieger Institute, my colleagues and I took a closer look at what happens in the brain when people are faced with decisions while mentally exhausted. This feeling of cognitive fatigue is not limited to a feeling of physical fatigue. Our team has demonstrated that this has a profound impact on people’s decisions and their willingness to put in effort. We found that repeated mental effort can significantly decrease people’s motivation to engage in difficult tasks, and we identified the areas of the brain that cause this phenomenon.

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We monitored the brain activity of 28 people using functional magnetic resonance imaging (fMRI) as they first completed tasks requiring cognitive effort and then decided whether they were willing to expend more mental effort to earn money. Monetary rewards allowed us to precisely assess people’s motivation in different states of fatigue.

While in the brain scanner, participants answered questions designed to test their working memory, a form of memory that allows people to hold multiple ideas in the brain at the same time. Participants had to recall strings of letters, with varying degrees of difficulty. After warming up and somewhat exhausting our participants with these tasks, we gave them the choice between an easy memory test with a $1 payout if they succeeded, or a more difficult memory test in which they would earn up to $8 if they answered correctly. As participants became more cognitively fatigued, they were much less likely to choose the more demanding option, even if it might pay more. This decision allows us to understand how cognitive fatigue can lead people to prioritize possible rewards over greater effort.

So what was going on in an exhausted brain that was making us behave differently? It turns out there is a decision-making brain system that specifically takes fatigue levels into account. As people completed increasingly grueling memory tasks, an area at the front of the brain called the dorsolateral prefrontal cortex sent stronger and stronger signals to the right anterior insula, a region in the middle-right of the brain that is known for monitoring how the body feels, including fatigue levels. Based on the patterns we observed, signals between these two brain areas were used to calculate a person’s willingness to exchange cognitive effort for reward. As people became mentally exhausted, activity between these two brain regions increased, meaning more and more signals were sent between the dorsolateral prefrontal cortex and the right anterior insula. This greater connectivity between brain regions predicted that a person would be less likely to take on the more difficult memory tasks.

These findings can help us examine cognitive fatigue, both as it is experienced in daily life and in certain conditions in which its effects are most pronounced, such as depression, long COVID, and multiple sclerosis. Much is still unknown about severe mental exhaustion, but our research offers a way to learn more. For example, it is possible that people with depression approach decision-making differently when experiencing cognitive fatigue. And identifying specific neural circuits linked to cognitive effort in depression and other conditions could eventually lead to the development of drugs and therapies targeting these brain pathways.

Together, the brain systems we studied help determine whether a reward is worth it. These fatigue signals likely evolved to prevent us from overloading our cognitive and physical systems. When fatigue signals are amplified, so are the costs of exerting more energy.

But there are ways to push ourselves, even when we’re tired, and situations in which that extra effort is worth it. Cognitive fatigue can often prevent us from doing activities that can replenish our mental resources, like taking a walk in the sun after work. And even after a long day at work or caring for loved ones, there are still demanding but important tasks we may need to accomplish, like making healthy food choices. Knowing the importance of an activity Or I find it interesting can help us overcome some feelings of fatigue, research shows. In other words, thinking about these activities in a way that reframes what you’re doing and why it’s important can help you take it one step further.