Working the night shift may increase the risk of developing ovarian cancer, according to a study by Hutchinson Center researchers published in the April issue of the journal Occupational and Environmental Medicine.
The researchers, led by Dr. Parveen Bhatti of the Public Health Sciences Division, observed that night shifts were associated with a 24 percent increased risk of invasive epithelial ovarian cancer and a 48 percent increased risk of borderline disease compared with those who only worked daytime hours. Only women ages 50 and above were significantly more likely to develop ovarian cancer if they had worked at night.
The authors, including Dr. Mary Anne Rossing—who originally launched this large-scale study of ovarian cancer—and Dr. Kristine Wicklund (both of PHS), based their findings on 1,101 women with invasive epithelial ovarian cancer and 389 patients with borderline disease, comparing them to 1,832 women without ovarian cancer. The women, between the ages of 35-74, were asked about the hours they worked, including whether they had ever worked night shifts. Women were also asked if they preferred being active at night ("owls") or during the day ("larks").
A greater proportion (27 percent) of women who described themselves as owls had worked night shifts than women (20 percent) who were self-described larks. Interestingly, the risk of advanced ovarian cancer was slightly higher among larks than owls, although this difference was not statistically significant. Findings were similar for borderline tumors. Previous research has found owls are likely to suffer less from night-shift work than larks in human populations.
The International Agency for Research on Cancer has classified shift work that disrupts the body's normal time clock (circadian rhythm) as a cancer-causing agent.
The authors said the suggestive evidence is consistent with an increased breast cancer risk associated with night-shift work found in other studies, but they did not find an increased cumulative risk for ovarian cancer the longer a woman had worked nights.
The researchers speculate the increased risk could be linked to melatonin, a powerful hormone that is normally produced at night, but is suppressed by ambient light, and which regulates reproductive hormones, particularly estrogen. Melatonin also scavenges harmful free radicals and boosts production of other antioxidants in the body.
The researchers also found use of oral contraceptives was lower among women with ovarian cancer, who also tended to have had fewer children than those without the disease. Oral contraceptives and childbearing are known to lower the risk of ovarian cancer.
The National Cancer Institute funded the study.
[Adapted from a BMJ Journals news release]