Working long hours comes with a slew of health issues, from too much stress to disturbed sleep, heart conditions, and mental-health disorders like anxiety and depression.

It may even cause changes in the brain, according to a new report published in Occupational and Environmental Medicine. Researchers from Korea found that people who regularly work long hours had significant brain differences compared to people who worked less.

Wanhyung Lee, from the department of preventive medicine at Chung-Ang University in Seoul, and his team studied 110 health care workers—some of whom worked more than 52 hours a week, which under Korean law constitutes overwork, and some of whom worked less. All had MRIs that allowed the scientists to analyze differences in the volume and concentration of certain brain tissues.

People who were overworked showed changes in 17 brain regions compared to those who worked typical hours. These differences included areas responsible for executive functions like logical reasoning, as well as managing emotions.

Lee says the results surprised him—in part because they suggest that the brain changes in response to stress and anxiety, with some potential negative consequences. “We anticipated that prolonged stress from overwork would affect brain structure, but finding increased volume in certain brain areas was somewhat unexpected,” he wrote in an email to TIME. “Our results suggest a potential neuroadaptive response, meaning the brain might initially try to compensate for increased cognitive and emotional demands. These surprising findings underscore the complexity of how the brain responds to prolonged occupational stress.”

Advances in brain imaging now make it possible to detect even small volume differences, Lee says. “These technological breakthroughs have empowered researchers to explore previously invisible biological changes induced by prolonged stress or excessive workloads, thus opening up an entirely new dimension in occupational and environmental health research.”