The European Heart Journal studied 6000 British civil servants and followed them for 11 years. They found that working an extra 3-4 hours a day is associated with increased coronary heart disease. The researchers controlled and adjusted for lifestyle, cardiac risk factors and other factors that would skew the results and still found that people who worked 3-4 extra hours a day had a 60% increase in risk for heart disease.
These results were for both women and men (ages 39-61), and the outcome measure was fatal myocardial infarction (heart attack) and non-fatal MI and proven angina. Other risk factors like smoking, elevated lipids, and diabetes made no difference in the results.
The conclusion: "Overtime work is related to increased risk of incident coronary heart disease independently of conventional risk factors. These findings suggest that overtime work adversely affects coronary health."