The question of whether frequent exposure to light-at-night increases breast cancer risk in women – either from exposures in occupational settings, such as night shift work, or at home – remains unresolved. But a study led by epidemiologists from the Department of Preventive Medicine at Stony Brook University School of Medicine, which appears on-line in the American Journal of Epidemiology, provides new data on the topic.
The study is one of only two published studies in the United States to investigate possible links between breast cancer and light-at-night from both occupational and residential exposures. Results are based on the responses from 576 women with breast cancer and 585 population-based controls.
The investigators observed no significant relationships between breast cancer and overall shiftwork or evening shiftwork alone. In fact, contrary to the results from other studies, there was a reduced disease risk for women working overnight shifts (45% reduction). Yet, in a small subset of women who most frequently woke up and turned on lights at home during usual sleeping hours – at least twice per week and twice per night – researchers found a positive association with breast cancer (65% increase), a new result that needs confirmation.
“We urge caution in interpreting the suggested association between breast cancer and a high frequency of light-at-night exposure at home during sleeping hours,” says Principal Investigator M. Cristina Leske, M.D., M.P.H., Distinguished Professor of Preventive Medicine, internationally recognized for her epidemiologic research, and a member of the prestigious Institute of Medicine, National Academy of Sciences. “We asked about past sleeping habits. However, the increased frequency of waking up and turning on lights among some women with breast cancer could reflect recent patterns after their diagnosis, thus leading to positive findings,” explains Dr. Leske.
Dr. Leske says the underlying hypothesis is complex to study. Prolonged exposure to light-at-night could increase breast cancer risk because it may lower levels of melatonin, a hormone affecting estrogen levels and tumor growth. She and co-investigators emphasize that their overall findings provide mixed evidence on the possible influence of light-at-night on breast cancer and do not strongly support the hypothesis of a relationship.
Study participants included women diagnosed with breast cancer from August 1996 to June 1997 and women without breast cancer, who had been living in the same Long Island (Nassau and Suffolk Counties), N.Y., home for 15 years or more, and were younger than 75 years. An in-person interview with each participant ascertained light-at-night exposure histories through shift work and at home. Questions on employment and shift work history included frequency of each job (days per week or otherwise), duration (months or years), and type of shift work (evening or overnight). Other questions covered hours slept, frequency of turning on lights during sleeping hours, and length of time a light was on.
The light-at-night and breast cancer investigation stems from the Electromagnetic Fields and Breast Cancer on Long Island Study (EBCLIS), a comprehensive study funded by the National Cancer Institute and National Institute of Environmental Health Sciences. The same research team concluded that EBCLIS results yielded no indication that residential magnetic fields increase the risk of breast cancer.
Co-authors from Stony Brook University included Principal Investigator M. Cristina Leske, M.D., M.P.H., Co-Principal Investigator Elinor Randi Schoenfeld, Ph.D., Erin S. O’Leary, Ph.D., Kevin Henderson, B.Sc., Roger Grimson, Ph.D., and Geoffrey Kabat, Ph.D.