This scenario will feel 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. By the end of the day, you finally take a moment to think about what to make them for dinner, and with it, whether you have the ingredients, how long it will take and whether your kids will even eat it. After an exhausting day, even simple tasks like making a meal can feel overwhelming.
That sensation is cognitive fatigue, and it can come up in all aspects of life. It’s the reason that an office worker performing 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, demanding shift might opt against making one extra patient visit despite initial plans to check on someone.
In research from 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 sense of cognitive fatigue isn’t just about feeling physically tired. Our team has demonstrated that it deeply impacts people’s decisions and willingness to exert effort. We found that repeated mental exertion can significantly decrease people’s motivation to engage in challenging tasks, and we pinpointed the areas of the brain that make this happen.
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We monitored brain activity in 28 people using functional magnetic resonance imaging (fMRI) while they first performed cognitively effortful tasks and then decided whether they were willing to exert more mental effort to earn some money. The monetary rewards allowed us to put an exact dollar amount on 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 one time. Participants were asked to recall strings of letters, with varying degrees of difficulty. After we’d gotten our participants warmed up and a bit worn out with these tasks, we gave them a choice between an easy memory test with a $1 payout if they succeeded, or a harder memory test in which they’d earn up to $8 for answering correctly. As participants became more cognitively fatigued, they were much less likely to choose the more demanding option, even though it could pay more. This decision let us understand how cognitive fatigue can make people value possible rewards versus greater effort.
So what was happening in an exhausted brain that makes us behave differently? It turns out there’s a decision-making brain system that specifically takes levels of fatigue into account. As people completed ever-more-exhausting memory tasks, an area at the front of the brain called the dorsolateral prefrontal cortex sent out increasingly strong signals to the right anterior insula, a region toward the middle-right of the brain that is known for monitoring how the body is feeling—including levels of fatigue. Based on the patterns we were seeing, signals between these two brain areas were used to compute a person’s willingness to trade cognitive effort for reward. As people got more mentally exhausted, the activity between these two brain regions increased, meaning more and more signals were sent between the dorsolateral prefrontal cortex and right anterior insula. That greater connectivity between brain regions predicted a person being less inclined to take on the tougher memory task.
These findings can help us examine cognitive fatigue, both as it’s experienced in daily life and in certain conditions in which its effects are most pronounced, such as depression, long COVID and multiple sclerosis. Much remains unknown about severe mental exhaustion, but our research offers a way of learning more. For example, it’s possible that people with depression might approach decision-making differently when experiencing cognitive fatigue. And identifying specific neural circuits related to cognitive effort in depression and other conditions could eventually also lead to the development of medications and therapies that target these brain pathways.
Together, the brain systems we have studied help determine whether a reward feels worth the effort. These fatigue signals have likely evolved to protect us from overtaxing our cognitive and physical systems. When fatigue signals are magnified, so, too, 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 push is worthwhile. Cognitive fatigue can often stop us from performing activities that might replenish our mental resources, such as taking a walk in the sunshine after work. And even after a long day at work or taking care of loved ones, there are still effortful but important tasks we may need to tackle, such as making healthy meal choices. Knowing the importance of an activity or finding it interesting can spur us past certain feelings of fatigue, research has shown. In other words, thinking about these activities in ways that reframe what you’re doing and why it matters can help you go that extra step.
