In 2007, International Agency for Research on Cancer (IARC), an agency of World Health Organization (WHO), concluded that night shift work is likely to raise the risk of cancer, particularly breast cancer. A recent review of over 1.4 million women challenges this conclusion, after revealing night shift work had minimal or no effect on breast cancer occurrence.
Research co-author Dr. Ruth Travis and colleagues published their findings in the Journal of the National Cancer Institute.
It is well established that shift work can affect the body’s circadian rhythm – the behavioral, mental and physical modifications that take place over a 24 hour cycle, which primarily dependson light and darkness in the environment.
Circadian rhythm interruption has been associated with an array of healthproblems, which includes sleep problems, obesity, diabetes, depression, and bipolar disorder.
The WHO conclusion was basedon a review of previous study that assessed how alterations in the circadian rhythm might affect breast cancer risk in animal models; at that time, there was minimal evidence on how night shift work might affect breast cancer risk in humans.
Commenting on their study Dr. Travis said,
“The majority of the human research accessible at that time wasretrospective in design, comparing responses from women already diagnosed with breast cancer with those from unaffected women”. Some retrospective outcomes might have been biased by differences in recall, and/or by differential participation in the research between women who have and haven’t worked at night.”
“Data from a number of large prospective studies [is] required to offer straight answers on connections with potential risk factors for cancer, such as night shift work.”
Evaluating new data on night shift work-breast cancer connection
Studies on how night shift work may impact women’s risk of breast cancer have been carried out by Dr.Travis and colleagues for almost a decade. The investigators analyzed data from 3 huge U.K. studies, which includes 522,246 individuals 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 offered details on their working patterns, and 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 several years – Showed no greater risk of breast cancer.
Further, the research team combined the data from the 3 U.K studies with data from 7 published studies that assessed the connection between night shift work and breast cancer.
No higher breast cancer risk, even with 3 decades of night shift work
Altogether, the 10 research involved a total of 1.4 million women. Among women who reported ever working night-shifts, 4,660 cases of breast cancer were reported.
Overall, compared with women who did not report working night shifts, the relative risk of breast cancer was
- 99 for women who had ever worked night shifts,
- 01 for women who had worked night shifts for at least 2 decades, and
- 00 for women who had worked night shift for 3 decades or more.
The investigators conclude that 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 don’t work night shifts.
Concluding their findings Dr. Travis said,
“The totality of the prospective evidence, reveals that night shift work, which includes long term night shift work, has minimal or no effect on breast cancer risk”. There are well established short-term risks of night shift work, such as fatigue. On the other hand possible long-term consequences of shift work, apart from breast cancer risk, remain unclear.”