A large study that tracked thousands of nurses over decades suggests many years of night-shift work may raise a woman’s risk of dying from heart disease or certain cancers.

The women on rotating night shifts for six years and longer were 19-23 percent more likely to die from cardiovascular disease and 11 percent more likely to die of any cause than those with little or no night-shift work. Women with 15 years or more on rotating night shifts also had a 25 percent increased risk of lung cancer death.

“I think the important message in this study is really the longer you work rotating night shifts in your career, your risks for developing cardiovascular disease or cancer increase,” said Carol Landis, an expert in sleep and health at the University of Washington School of Nursing in Seattle.

“If you think about it, it’s really minimal exposure (only three or more rotating shifts in a month),” Landis, who was not involved in the study, told Reuters Health.

Past research has tied late-night work hours to increased risks of type 2 diabetes, heart disease and cancer. The authors of the new study in the American Journal of Preventive Medicine also point to observed associations between night shifts and high blood pressure, chronic fatigue, sleep problems and excess weight.

The body’s circadian “clock” system is believed to influence many aspects of health, and the sleep-regulating hormone melatonin is thought to have antitumor effects, the authors note. So it’s plausible that night shift hours could disrupt melatonin and disturb systems that control heart rate, inflammation and metabolism of blood sugar and fats, they write.

“I became interested in studying the health impact of night work on health during my residency and clinical fellowship, when I began to suffer from near-to-constant sleep deprivation and feeling jet-lagged,” said the study’s senior author Dr. Eva Schernhammer of Brigham and Women’s Hospital and Harvard Medical School in Boston. “Simultaneously, I observed the many different health issues my colleagues who worked rotating night shifts had to cope with.”

Schernhammer’s team used data from the Nurses’ Health Study, and included nearly 75,000 female registered nurses in their analysis. The researchers followed the women from 1988 to 2010, at which point 14,181 women had died.

The team then looked at the women’s weight, diets, lifestyles and other factors as well as their causes of death.

Overall, women who worked rotating night shifts for longer periods were older (mean age 66) and heavier, but more physically active than others. They were also more likely to smoke and less likely to take postmenopausal hormones or multivitamins. They tended to drink less alcohol and eat less daily cereal fiber and were more likely to have diabetes, high blood pressure and high cholesterol.

Among women who had done 15 or more years of night shifts at least three nights per month, there were 1,430 deaths, 364 from cardiovascular disease and 506 from cancers (150 from lung cancer).

“We examined overall mortality in these women, and observed significantly higher overall mortality, as well as higher mortality from cardiovascular disease in women with several years of rotating night shift work, compared to nurses who had never worked night shifts,” Schernhammer said in an email.

The results held even after they were adjusted for weight, smoking and other factors that might explain the link between shift work and disease.

Landis pointed out that one limitation of the study was that all the nurses were married, so the results might not be applicable to younger, single women.

“What really struck me about the study was it’s a really large sample and they did a very nice job of controlling variables, so the risk even though it’s modest, is over and above what other risk factors exist,” Landis said.

Women who need to work night shifts should avoid smoking cigarettes, be sure to eat a nutritional diet high in fiber and reduce light exposure when they get off work in the morning, Landis said.

“So if they wear dark glasses, even if the sun is not out, if they take melatonin, that’s shown to be very good for helping night shift workers to sleep during the day . . . they should really follow a very specific schedule and probably better to not go to sleep right away and be sure to get at least six to eight hours of sleep,” she said.