Night shift work may harm women's heart health

Rotating night shift work may take a toll on women's hearts, new research shows. The study, published in JAMA, found that female nurses who had a history of working night shifts in conjunction with daytime shifts had a slightly higher risk of coronary heart disease than those who did not.

"Overall, we found a small and modest relationship between rotating night shift work and new development of coronary heart disease," lead author Dr. Celine Vetter, associate epidemiologist in the Channing Division of Network Medicine, and chronobiologist at Brigham and Women's Hospital, told CBS News. "This was after taking into account traditional risk factors for CHD."

The results also suggests that recent night shift work might be most harmful to heart health and that longer time since stopping shift work was associated with a decreased risk.

For this study, Vetter and her team analyzed 24 years worth of data on about 189,000 women who participated in the Nurses' Health Study I and II. These women reported their lifetime exposure to rotating night shift -- defined as three of more night shifts per month -- in addition to day and evening shifts. They also answered questions about their lifestyle, including information on their diet, exercise habits, and whether or not they smoked cigarettes.

Additionally, the researchers had the participants report any adverse heart events they experienced, including whether they had an angiogram that confirmed heart disease-related chest pain, a heart attack, or cardiovascular procedures such as angioplasty, coronary artery bypass graft surgery, or stents. The study authors confirmed self-reported heart attacks and deaths resulting from heart disease with information in medical records and death certificates.

Over the 24-year period, more than 10,000 newly developed cases of coronary heart disease occurred. The analysis showed that women who reported working more than 10 years of rotating night shifts had a 15 to 18 percent increased risk of developing coronary heart disease than those who did not.

The researchers controlled for known risk factors for heart disease, including smoking, poor diet, low levels of physical activity, and elevated body mass index (BMI).

Though the study does not prove that rotating night shift work actually causes heart disease, the results are in line with previous findings, Vetter notes.

The study also doesn't look at the potential reasons for the link, but researchers think the answer lies in the disruption in the circadian rhythm, which leads to sleep deprivation.

"We know from very carefully conducted laboratory studies that this will lead to increased inflammation, can disrupt the metabolism, and suppress the immune system," Vetter said. "There's also social stress that comes with night shift work because you have to organize your private and family life around that schedule. All of this together is thought to be the underlying mechanism of this association."

She also notes that individual characteristics, including variations in sleep patterns and quality, as well as an individual's biological rhythm, might affect risk.

Future research, Vetter said, should look at these individual traits, as well as information on exact start and end times to see if different schedules carry different risks.

Vetter emphasized that the overall risk is very small and said that individuals who currently work rotating night shifts should focus on reducing their heart disease risk by adjusting known lifestyle risk factors.

"This is a modest association with a lot of unanswered questions," she said. "If you stop smoking, eat a healthy diet, have a healthy body weight and if you try to be active, that's the key to coronary health."