Cognitive fatigue: How the brain decides it’s time to slow down

Mental fatigue is one of the most universal experiences of modern life. After hours of focusing, learning, or handling repetitive tasks, a subtle heaviness sets in and motivation begins to wane. This phenomenon is more than just an unpleasant sensation, it reshapes the way the brain evaluates the efforts we’re willing to make.

Contrary to the widespread belief that a fatigued brain simply “performs worse,” cognitive performance may remain stable, or even improve, thanks to compensation mechanisms. What actually changes is how effort is valued. The brain constantly recalibrates the balance between cost and benefit, and fatigue acts like an invisible inflation: each task feels more demanding than before. In other words, it’s not a depletion of resources, but a subtle shift in the cost-benefit analysis that guides our choices.

Neuroimaging studies reveal that this recalibration involves specific brain regions. The dorsolateral prefrontal cortex, responsible for tasks involving memory and attention, signals a rising workload. Meanwhile, the insula, which converts internal states into subjective feelings, translates those signals into immediate perceptions: “How much will this cost me, here and now?” As fatigue builds up, the insula amplifies perceived effort, making demanding options less appealing, even if they come with greater rewards.

To further investigate how the brain recalibrates effort under fatigue, Grace Steward and Vikram Chib from Johns Hopkins School of Medicine designed an experiment to directly test the impact of cognitive fatigue on decision-making. Their goal was to observe not only what participants reported feeling, but also how the brain converted that state into choices, whether to engage in or avoid demanding tasks.

Participants completed a classic working memory task known as the n-back, which requires remembering sequences of letters and identifying those that reappear after a certain interval. The higher the “n” level, the greater the mental effort required. To make difficulty instantly recognizable, each level was assigned a specific color. Participants then chose between two options: a simple, low-reward task (the default choice) or a more demanding task with a higher payoff. These decisions were recorded using functional MRI, first in a rested state, then after repeated task blocks designed to induce fatigue.

The findings revealed that, even when performance remained stable, participants reported growing fatigue, which gradually influenced their decisions. They were increasingly inclined to select the easier task, even at the expense of higher rewards. Brain scans confirmed this shift: activity in the dorsolateral prefrontal cortex increased with task load, while the insula, more attuned to subjective value, intensified its weighting of effort cost. Connectivity between the two regions also increased, as if the brain began factoring its own fatigue state more heavily before committing to further effort.

Fatigue as a safety switch

These findings suggest that cognitive fatigue is not a failure but an adaptive strategy. It acts as a regulatory signal, increasing the perceived cost of effort to prevent overcommitment and preserve long-term balance. The brain isn’t just designed to maximize output, it recalibrates to avoid overload and ensure stability.

Under fatigue, some brain regions work harder to maintain performance, but this compensation comes at a cost: it heightens the subjective sense of exhaustion. At the same time, motivational circuits connecting the prefrontal cortex to the striatum become less reliable, disrupting our ability to accurately assess the value of sustained effort. Fatigue, then, is not a loss of capacity, it’s a reprioritization: protect the organism first, even if it means lowering motivation for demanding tasks.

Cognitive fatigue isn’t a wall that stops us abruptly; it’s more like a yellow light, signaling that it’s time to slow down. It reveals that the brain has a sophisticated system of self-regulation, capable of adjusting mental investment based on available resources. The study by Steward and Chib (2024) beautifully illustrates this internal dialogue between the prefrontal cortex and the insula, two regions that collaborate to redirect our choices toward more accessible options when fatigue becomes overwhelming.

Seen through this lens, cognitive fatigue is not a flaw to fix but a preservation strategy. It reflects the brain’s ability to continuously reassess the cost of effort to maintain a delicate balance between motivation, engagement, and well-being.