The research, spearheaded by Allard Dembe, an Ohio State University professor of health services management and policy, was based on data from interviews with almost 7,500 women who were part of the National Longitudinal Survey of Youth 1979, which tracked adults born between 1957 and 1964 over a 32-year period.

Dembe and his colleagues found that women who worked an average of 60 hours or more over the three decades of the study had three times the risk of getting diabetes, cancer, heart trouble and arthritis than those who worked 40-hour weeks.

Men weren’t nearly as affected by long work hours, the researchers found: They had a higher incidence of arthritis, but none of the other chronic diseases. Moreover, men who worked 41-50 hours per week had a lower risk of heart disease, lung disease and depression than those who worked fewer than 40 hours.

Dembe tells Fortune that he was “so, so surprised by the real gender difference,” noting that he has never before seen such a pronounced divide between men’s and women’s results in his research.

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Why do women’s bodies respond so much more dramatically to longer hours? The answer may have to do with all the responsibilities they shoulder at home. “My speculation is that women have multiple roles to maybe a greater extent than men,” Dembe says. “It may just be that they don’t have time to take care of themselves” because they spend much of their time outside of work on caretaking.

Another possibility is that women’s bodies are simply not as able as men’s to withstand long hours, says Dembe, though he notes that both of these explanations are simply unproven hypotheses.