A recent study from the IMT School of Advanced Studies Lucca provides new insight into how mental fatigue affects our behavior and decision-making, revealing physical brain changes that may explain a tendency toward hostility and poor choices when we are mentally exhausted.
Published in Proceedings of the National Academy of Sciences (PNAS), this research combines neuroscience and economics to explore “ego depletion”—the idea that willpower diminishes with use.
Ego depletion theory emerged in the early 2000s, suggesting that self-control is a limited resource that, once used, becomes temporarily “depleted.”
This theory gained traction as it helped explain why people may become less empathetic, altruistic, or patient when under mental strain.
However, more recent studies questioned this theory, showing inconsistent results and failing to replicate the supposed effects of willpower depletion. The new study aimed to settle the debate by identifying specific brain activity patterns associated with mental fatigue.
One phenomenon, known as “local sleep,” occurs when certain areas of the brain, despite being awake, show signs of sleep-related activity.
Research on sleep had previously found that when people are mentally fatigued, certain areas of the brain start producing sleep-like electrical waves known as delta waves. The research team suspected that these waves might be the neurological basis for ego depletion.
To test this, the researchers had participants complete a challenging one-hour task designed to cause mental fatigue.
Afterward, they played economic games requiring cooperation or competition, including a “hawk and dove” game where players decide between cooperative or aggressive strategies to divide limited resources.
The fatigued participants were compared with a control group who didn’t perform the tiring tasks.
The results were striking: mentally fatigued participants were much less cooperative. In the “No Fatigue” group, 86% chose peaceful cooperation, whereas in the “Fatigue” group, this rate dropped to 41%.
Electroencephalogram (EEG) readings taken during these games revealed that the fatigued participants showed sleep-like delta waves in the frontal cortex, an area responsible for decision-making, while this activity was absent in the control group.
Erica Ordali, the study’s lead author, explained that these findings suggest mental fatigue triggers local sleep in specific brain areas, which could impair judgment and increase irritability.
Co-author Pietro Pietrini added that the findings lend scientific support to the common advice to “sleep on it” before making decisions.
Mental exhaustion appears to affect brain areas critical for weighing choices, increasing the likelihood of decisions that may run counter to one’s best interest.
These findings hold broad implications, suggesting that people may make more impulsive, hostile, or poorly considered choices when mentally drained.
This has relevance not only for daily life decisions but also for areas like economic negotiations or legal agreements, where decisions under fatigue could lead to negative outcomes.
In summary, this multidisciplinary study shows that mental fatigue isn’t just an abstract feeling—it causes physical changes in the brain that make us more likely to act uncooperatively and against our own interests.
This insight could help inform strategies to avoid poor decision-making by managing fatigue and considering rest before important decisions.
If you care about depression, please read studies about how dairy foods may influence depression risk, and B vitamins could help prevent depression and anxiety.
For more information about mental health, please see recent studies that ultra-processed foods may make you feel depressed, and extra-virgin olive oil could reduce depression symptoms.
The research findings can be found in PNAS.
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