People who work a mix of day and night shifts may face a greater risk of dying from heart disease than those who work fixed days or nights only, a new study from Japan suggests.

Also, the effect of heart disease risk factors such as high blood pressure, obesity, excessive alcohol consumption and cigarette smoking was even greater for individuals who worked rotating shifts, Dr. Yoshihisa Fujino and his colleagues report in the American Journal of Epidemiology.

Fujino of the University of Occupational and Environmental Health in Kitakyushu and his team looked at 17,649 men aged 40 to 69 participating in a study of cancer risk. Nearly 84 percent chiefly worked days, while about 5 percent worked nights only, and roughly 11 percent were on rotating shifts.

Over a period of about 13 years, 1363 of the men died, with 86 of these deaths due to heart disease.

Men who worked rotating shifts were 60 percent more likely than those who worked day shifts to have diseases of the heart and blood vessels, and they were 2.32 times more likely to die of heart disease such as a heart attack.

And if men had other risk factors for heart disease, the effect of rotating shift work was even stronger. For example, rotating shift workers with high blood pressure were 3.4 times more likely to die of heart disease than day workers with high blood pressure.

However, men who worked fixed nights were at no greater risk for heart disease or death from related causes than those who worked days only.

Past studies have suggested that difference in health among people working different shifts may be due to differences in heart disease risk factors - for example, people who work nights may be more likely to smoke than those who work days. However, in the current study, the effect of rotating shift work persisted even after the researchers used statistical techniques to adjust for such risk factors.

Shift work is known to affect circadian rhythms and body functions such as blood pressure, heart rate and hormone secretion, Fujino and his colleagues note. It may be more difficult people’s circadian rhythms to adapt to a rotating shift than to steady nighttime work, they suggest.

SOURCE: American Journal of Epidemiology, July 15, 2006.