Women who work rotating night shifts for 5 years or more may be at increased risk of all-cause and cardiovascular disease mortality, while working such shifts for 15 years or more may raise the risk of lung cancer mortality. This is according to a new study published in the American Journal of Preventive Medicine.
An array of past research has linked shift work to greater risk of illness. Last July, Medical News Today reported on a study associating shift work with increased risk of type 2 diabetes, while a 2012 study found shift work may raise the risk of heart attack and stroke.
Shift work has been strongly associated with greater risk of cardiovascular disease (CVD) and cancer. A 2013 study, for example, found women who work night shifts may be more likely to develop ovarian cancer. And in 2007, the World Health Organization (WHO) even classed night shift work as a potential carcinogen because it disrupts the body's circadian rhythm.
This latest study, according to the international team of researchers, adds to the increasing evidence that shift work may be detrimental to health and longevity.
To reach their findings, the team analyzed data of almost 75,000 women from the Nurses' Health Study - a cohort of 121,700 female nurses aged 30-55 years that began in 1976.
In 1988, 85,197 women in the cohort responded to questionnaires about their patterns of rotating night shift work - defined as working 3 nights or more each month, as well as working days or evenings in the same month.
The women were also asked how long they had engaged in such working patterns: 1-2 years, 3-5, 6-9, 10-14, 15-19, 20-29 or 30 years or more.
The researchers excluded women who had pre-existing CVD or cancer (except non-melanoma skin cancer), leaving 74,862 women to be included in the study.
Rotating night shift work may raise risk of all-cause mortality by 11%
The results of the study revealed that women who had worked rotating night shifts for 5 years or more were at higher risk of all-cause and CVD mortality than those who had not worked such shifts or who had worked them for fewer years.
In detail, the team found that that women who had worked rotating night shifts for 6-14 years or 15 years or more were 11% more likely to die from all causes. Women who had worked rotating night shifts for 6-14 years were at 19% greater risk of CVD mortality, while this risk was 23% higher for women who worked such shifts for 15 years or more.
In addition, the team found that women who worked rotating night shifts for 15 years or more were at 25% higher risk of lung cancer. No association was found between rotating shift work and increased mortality from any other cancer.
Commenting on the team's findings, study author Dr. Eva S. Schernhammer, associate professor of medicine at Harvard Medical School and associate epidemiologist at Brigham and Women's Hospital - both in Boston, MA - says:
"These results add to prior evidence of a potentially detrimental relation of rotating night shift work and health and longevity.
[...] To derive practical implications for shift workers and their health, the role of duration and intensity of rotating night shift work and the interplay of shift schedules with individual traits (e.g., chronotype) warrant further exploration."
Dr. Schernhammer notes that this study is one of the largest in the world to incorporate such a high number of rotating night shift workers and to monitor them over a long period - 22 years. In addition, she says the results are strengthened by the fact the study only included nurses.
"A single occupation provides more internal validity than a range of different occupational groups, where the association between shift work and disease outcomes could be confounded by occupational differences," adds Dr. Schernhammer.
In November last year, MNT reported on a study published in the journal Occupational & Environmental Medicine, which suggested shift work may impair brain function.