Mental exhaustion after intense cognitive work is real—and now we have brain scans to prove it.

In a 2022 study, neuroscientists used magnetic resonance spectroscopy to analyze chemical changes in the brain as young adults carried out demanding cognitive tasks like classifying letters as vowels or consonants over a grueling, 6-hour period. As the day wore on, “potentially toxic by-products” accumulated in the participants’ prefrontal cortex—the region of the brain responsible for executive function skills like decision-making, planning, and attention.

Eventually, the toxins triggered “a metabolic regulation mechanism”—akin to a car’s “maintenance required” light—that signaled the brain to shift gears, redirecting energy away from higher-order cognitive tasks in favor of activities requiring less cognitive effort.

That feeling of being mentally exhausted after a day of thinking hard? It’s not a mirage, the neuroscientists say—it’s a biochemical nudge urging us to shelve important decisions until we’ve had a good rest.