Working night shifts has little or no impact on women's risk of breast cancer, a new study suggests.
Study co-author Dr. Ruth Travis, of the Cancer Epidemiology Unit at Oxford University in the United Kingdom, and colleagues publish their findings in the Journal of the National Cancer Institute.
According to the Centers for Disease Control and Prevention (CDC), around 15 million adults in the United States work full-time night shifts, rotating shifts, or other irregular schedules.
It is well established that such working patterns can disrupt the body's circadian rhythm - the physical, mental, and behavioral changes that occur over a 24-hour cycle, which mainly respond to light and dark in the environment.
Circadian rhythm disruption has been associated with an array of health problems, including sleep disorders, obesity, diabetes, depression, and bipolar disorder. And in 2007, the International Agency for Research on Cancer (IARC) - an agency of the World Health Organization (WHO) - concluded that night shift work is "probably carcinogenic to humans."
The conclusion was based on a review of previous research that assessed how disruption to the circadian rhythm might impact the risk of breast cancer in animal models; at that time, there was limited evidence on how night shift work might affect breast cancer risk in humans.
"Most of the human studies available at that time were retrospective in design, comparing responses from women already diagnosed with breast cancer with those from unaffected women," Dr. Travis told Medical News Today. "Some retrospective results might have been biased by differences in recall, and/or by differential participation in the studies between women who have and haven't worked at night."
"Information from a number of large prospective studies [is] needed to provide reliable information on associations with potential risk factors for cancer, such as night shift work."
Assessing new data on night shift work-breast cancer link
Almost 10 years on, Dr. Travis and colleagues conducted a new review to estimate how night shift work might influence women's risk of breast cancer.
The researchers analyzed data from three large U.K. studies, including 522,246 participants from the Million Women Study, 251,045 women from UK Biobank, and 22,559 women from the European Prospective Investigation into Cancer and Nutrition-Oxford (EPIC-Oxford) study.
All subjects provided information on their working patterns, and they were followed-up for incidence of breast cancer.
Compared with women who did not work night shifts, those who did work night shifts - even those who had done so for many years - showed no higher risk of breast cancer.
Next, the team combined the data from the three U.K studies with data from seven published studies that assessed the link between night shift work and breast cancer.
No greater breast cancer risk, even with 30 years of night shift work
Altogether, the 10 studies included a total of 1.4 million women. Among women who reported ever working night shifts, 4,660 cases of breast cancer occurred.
Overall, compared with women who did not report working night shifts, the relative risk of breast cancer was 0.99 for women who had ever worked night shifts, 1.01 for women who had worked night shifts for at least 20 years, and 1.00 for women who had worked night shift for 30 years or more.
In conclusion, the researchers say their findings suggest women who work night shifts - regardless of how long they have followed such working patterns - have the same risk of developing breast cancer as women who do not work night shifts.
MNT asked Dr. Travis whether their findings should offer peace of mind for women who work night shifts.
"The totality of the prospective evidence, shows that night shift work, including long-term night shift work, has little or no effect on breast cancer risk," she replied. "There are established short-term risks of night shift work, including fatigue. However, possible long-term effects of shift work, besides breast cancer risk, remain unclear."