Nightshifts Linked to Increased Risk for Ovarian Cancer

Zosia Chustecka

March 15, 2013

Another study has shown a link between nightshift work and cancer. This time, an increase in the risk for ovarian cancer has been reported by American researchers.

Much of the previous work on the link between cancer and nightshifts has focused on breast cancer; there has even been a compensation case. In 2008, Denmark become the first country to pay government compensation to women who developed breast cancer after long spells of working at night or working at a job that disrupted circadian rhythms; many of the women were air hostesses and nurses.

The latest report, focusing on ovarian cancer, was published in the April issue of Occupational and Environmental Medicine.

This increase in the risk for ovarian cancer with nightshift work is consistent with, and of similar magnitude to, the risk for breast cancer, say lead author Parveen Bhatti, PhD, and colleagues from the epidemiology program at the Fred Hutchinson Cancer Research Center in Seattle, Washington.

Increase in Risk

The researchers examined data from a local population-based cancer registry that is part of the Surveillance Epidemiology and End Results (SEER) Program. They identified 1101 women with advanced epithelial ovarian cancer, 389 with borderline disease, and 1832 without ovarian cancer (control group).

The women, who were 35 to 74 years of age, were asked about the hours they worked, and specifically whether they had ever worked the nightshift.

The researchers found that 26.6% of the women with invasive cancer had worked nights at some point, as had 32.4% of those with borderline disease and 22.5% of those in the control group.

In the entire cohort, the median duration of nightshift work was 2.7 to 3.5 years. The most common types of nightshift jobs were in healthcare, food preparation and service, and office and admin support.

The researchers conclude that working nights is associated with an increased risk for both invasive ovarian cancer (odds ratio [OR], 1.24, 95% confidence interval [CI], 1.04 - 1.49) and borderline disease (OR, 1.48; 95% CI, 1.15 - 1.90).

The increased risk was restricted to women 50 years and older, and to serous and mucinous tumor histologies, they note.

Richard Stevens, PhD, professor of cancer epidemiology at the University of Connecticut in Farmington, who was asked to comment on the findings, said that "this is a very nice study, carefully done."

However, like all studies of this nature, there could be an overrepresentation of shift work in the control group if the information was gathered using telephone calls made during the day, he told Medscape Medical News. But this is a "minor point," he explained, and, if anything, would make the magnitude of the effect even larger.

Dr. Stevens disagrees with the researchers, who suggest that there is no effect of increasing duration of nightshift work. He said that their data do suggest a dose–response relation in the highest category of cumulative nightshift work-years studied (more than 7 years), although there were very few women in this group.

Risk Higher in "Larks" Than "Owls"

This researchers found that the magnitude of the risk for ovarian cancer in women who worked the nightshift varied by chronotype. Women who preferred mornings ("larks") were at higher risk than those who preferred evenings and nighttime ("owls"), but this differential effect was not statistically significant, they note.

Future studies should include detailed evaluation of these preferences, because studies of chronotype could help identify subgroups that are particularly sensitive to the carcinogenic effects of shift work, the researchers note.

In an accompanying commentary, Thomas Erren, MD, MPH, from the Institute and Policlinic for Occupational Medicine, Environmental Medicine and Prevention Research, University of Cologne, Germany, elaborates on this point.

"It seems intuitive that individuals who wake up very early in the morning (larks) and are most alert in the 8 to 16 hours that follow would also work in this time window with relative ease. Conversely, individuals who prefer to sleep in (owls) are most alert in late evening hours and can be expected to perform with greater ease in much later time windows," Dr. Erren writes.

The problem arises when work does not fit a person's chronotype, he continues. "Larks and owls experience more stress and strain when working outside their preferred time windows of activity," he explained.

"Ultimately, we may want to define shift work specifically for an individual as 'work at chronobiologically unusual times,' that is, the time window of work is not ready compatible or 'in phase with' the individual's 'time-of-day type,' he explains. "Such a paradigm shift in what we label as shift work...may become a necessary means to achieve 2 ends: to reliably research shift workers' possible health risks, including cancer, and to effectively prevent disease in working populations who differ chronobiologically as evidence by their chronotypes," he adds.

The authors have disclosed no relevant financial relationships.

Occup Environ Med. 2013;70: 231-237, 283-284. Abstract, Commentary