To the Editor:
Most of the published epidemiologic literature on the effect of light at night and cancer examines the risk of breast cancer (eg, Davis et al,1 Hansen2). A recent systematic review and meta-analysis concluded that increased risk of breast cancer may be associated with engagement in night work (a proxy for exposure to light at night) and a related decrease in melatonin production.3 Despite the biologic plausibility of decreased melatonin affecting cancers other than of the breast,4 there is only one published report of increased risk of colorectal cancer in women who work rotating night shifts5 and 1 recent report on prostate cancer risk in males.6 We assessed the association between rotating shift work and the risk of prostate cancer using previously collected case–control data that assessed medical history and sexual and physical factors.
Details of the population-based study have been reported elsewhere.7 Participants in this questionnaire-based study were 760 cancer registry-identified cases, age 45 to 84 years, who were diagnosed between 1995 and 1998 and 1,632 controls who were frequency-matched on age—all of whom resided in northeastern Ontario.
The 25-page mailed questionnaire included questions about age and family history of prostate cancer, medical and sexual history, physical characteristics, diet, physical activity, smoking, residential history, and lifetime work history. Specifically, for each job held for 1 year or more (including seasonal work and part-time work), respondents were asked number of years worked, work type (full-time, part-time, seasonal), type of industry/employer, name of employer, and usual work time (daytime shift, evening/nightshift, rotating shift, other). We created an exposure variable for ever having worked rotating full-time work and other quantitative metrics related to full-time rotating shift work as our surrogate for increased light at night. These included duration (years) of shift work, age at first shift work (based on the quartile distribution in controls), age at first shift work (based on age decades), and years since first shift work (based on the quartile distribution in controls). We adjusted all analyses for age and family history of prostate cancer.
Ever having worked full-time rotating shift work was associated with increased risk of prostate cancer, with an adjusted odds ratio (OR) of 1.2 (95% confidence interval [CI] 1.0–1.4) (Table 1). Compared with men who never worked full-time rotating shift work, men who worked full-time rotating shift work for less than 7 years were at highest risk of prostate cancer, with an OR of 1.4 (1.1–1.9). Men who began working full-time rotating shift work in their mid 20s appeared at highest risk, with an OR of 1.4 (1.1–1.8), and starting full-time rotating shift work 37 to 44 years before diagnosis was associated with a 34% increased risk (CI = 1.0–1.8).
These results suggest that working full-time rotating shifts may be associated with increased risk of prostate cancer. Given previous epidemiologic research showing shift work as a risk factor for breast and colorectal cancers in women, these results provide modest epidemiologic support for the association in males.
Our exposure variable was not originally designed to assess this hypothesis, and therefore we could not assess other aspects of exposure to light at night that might affect cancer risk. Nonetheless, although our exposure variable is a proxy, our results are interesting given that nondifferential misclassification of exposure would be expected to attenuate effects.
This exploratory analysis indicates that working full-time rotating shift work may be associated with an increased risk of prostate cancer, which is suggestive of an association between light at night and cancer risk. This finding should be examined in other studies.
Regional Cancer Program
Sudbury Regional Hospital
Sudbury, Ontario, Canada
Epidemiology Research Unit
Regional Cancer Program, Sudbury
Division of Human Sciences, Northern
Ontario School of Medicine
Sudbury, Ontario, Canada
Departments of Public Health Sciences and Nutritional Sciences
University of Toronto
Toronto, Ontario, Canada
Research Unit, Division of Preventive Oncology
Cancer Care Ontario
Toronto, Ontario, Canada
1.Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst
2.Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology
3.Megdal SP, Kroenke CH, Laden F, et al. Night work and breast cancer risk: a systematic review and meta-analysis. Eur J Cancer
4.Schernhammer ES, Schulmeister K. Melatonin and cancer risk: does light at night compromise physiologic cancer protection by lowering serum melatonin levels? Br J Cancer
5.Schernhammer ES, Laden F, Speizer FE, et al. Night-shift work and risk of colorectal cancer in the Nurses’ Health Study. J Natl Cancer Inst
6.Kubo T, Ozasa K, Mikami K, et al. Prospective cohort study of the risk of prostate cancer among rotating-shift workers: findings from the Japan collaborative cohort study. Am J Epidemiol
7.Lightfoot N, Conlon M, Kreiger N, et al. Medical history, sexual, and maturational factors and prostate cancer risk. Ann Epidemiol