Night Shift Work and the Risk of Endometrial Cancer

Abstract

Melatonin has several oncostatic properties, including possible anti-estrogenic and anti-aromatase activity, and seems to be linked with fat metabolism. Night workers have lower levels of melatonin, which may predispose them to develop cancer. Endometrial cancer risk is influenced significantly by hormonal and metabolic factors; therefore, we hypothesize that night workers may have an increased risk of endometrial cancer. Of the 121,701 women enrolled in a prospective cohort study, 53,487 women provided data on rotating night shift work in 1988 and were followed through on June 1, 2004. A total of 515 women developed medical record-confirmed invasive endometrial cancer. We used Cox regression models to calculate multivariate relative risks (MVRRs), controlling for endometrial cancer risk factors. Women who worked 20+ years of rotating night shifts had a significantly increased risk of endometrial cancer [MVRR, 1.47; 95% confidence interval (95% CI), 1.03-1.14]. In stratified analyses, obese women working rotating night shifts doubled their baseline risk of endometrial cancer (MVRR, 2.09; 95% CI, 1.24-3.52) compared with obese women who did no night work, whereas a nonsignificant increase was seen among non-obese women (MVRR, 1.07; 95% CI, 0.60-1.92). Women working rotating night shifts for a long duration have a significantly increased risk of endometrial cancer, particularly if they are obese. We speculate that this increased risk is attributable to the effects of melatonin on hormonal and metabolic factors. Our results add to growing literature that suggests women who work at night may benefit from cancer prevention strategies. [Cancer Res 2007;67(21):10618-22]

Characteristics of the Million Women Study participants who have and have not worked at night

“…We also found that ever night workers were more likely to have used exogenous hormones. Few other studies have reported on exogenous hormone use among night-working women, although ever use of HRT was found to be less common among rotating night shift workers in the Nurses' Health Study, particularly among long-duration night workers (15). Given the established effects of the reproductive and hormonal factors on the risk of breast cancer (16)(17)(18), the differences observed in the current study would put ever night workers, and especially long-duration night workers, at an increased risk of the disease.…”

Breast cancer among shift workers: results of the WOLF longitudinal cohort study

“…The IARC review further stated that the association between shift work and breast cancer was supported by sufficient evidence among experimental animals but only limited evidence in epidemiological studies on humans. Regarding cancer, shift work has been associated with breast cancer (5,6,7), prostate cancer (8), endometrial cancer (9), colorectal cancer (10), and Non-Hodgkin's lymphoma (11). The most extensively studied cancer in this context is female breast cancer.…”

Sleep Disorders in Shift Workers in the Emergency Department and Efficacy of Melatonin

“…A causal link between shift work and cancer has such strong supporting evidence that shift work has been classified as a "probable carcinogen" (19). Besides colorectal cancer, there are many reports about a relation between shift work and endometrial, prostate, and breast cancer (20)(21)(22). Exposure to light at nighttime suppresses melatonin production.…”

“…This treatment is effective, especially in elderly people; the response is at the moderate level in young individuals. For this purpose, the patient is exposed for 1-2 h (between 19:00 and 21:00) to bright blue light (2500-10000 lux) (20). If the patient needs to receive light during the daytime, he/she is exposed to 2500-5000 lux for 2 daytime hours for a duration of 4 weeks (Figure 1).…”