Rotating night shifts and risk of breast cancer in women participating in the nurses' health study.
BACKGROUND
Melatonin shows potential oncostatic action, and light exposure during night suppresses melatonin production. There is little information, however, about the direct effect of night work on the risk of cancer. We investigated the effect of night work in breast cancer.


METHODS
We examined the relationship between breast cancer and working on rotating night shifts during 10 years of follow-up in 78 562 women from the Nurses' Health Study. Information was ascertained in 1988 about the total number of years during which the nurses had worked rotating night shifts with at least three nights per month. From June 1988 through May 1998, we documented 2441 incident breast cancer cases. Logistic regression models were used to calculate relative risks (RRs) and 95% confidence intervals (CIs), adjusted for confounding variables and breast cancer risk factors. All statistical tests were two-sided.


RESULTS
We observed a moderate increase in breast cancer risk among the women who worked 1-14 years or 15-29 years on rotating night shifts (multivariate adjusted RR = 1.08 [95% CI = 0.99 to 1.18] and RR = 1.08 [95% CI = 0.90 to 1.30], respectively). The risk was further increased among women who worked 30 or more years on the night shift (RR = 1.36; 95% CI = 1.04 to 1.78). The test for trend was statistically significant (P =.02).


CONCLUSIONS
Women who work on rotating night shifts with at least three nights per month, in addition to days and evenings in that month, appear to have a moderately increased risk of breast cancer after extended periods of working rotating night shifts.
Schernhammer
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E. S. Schernhammer, Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, USA. eva.schernhammer@channing.harvard.edu
Laden
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Speizer
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Willett
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Hunter
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Kawachi
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Colditz
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D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D002940:Circadian Rhythm / Q000528:radiation effects*; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008027:Light / Q000009:adverse effects*; D008029:Lighting; D008550:Melatonin / Q000378:metabolism / Q000494:pharmacology; D008875:Middle Aged; D015337:Multicenter Studies as Topic; D009726:Nurses; D010870:Pineal Gland / Q000378:metabolism / Q000528:radiation effects; D017698:Postmenopause; D017697:Premenopause; D012306:Risk; D012307:Risk Factors; D012636:Secretory Rate / Q000528:radiation effects; D011795:Surveys and Questionnaires; D014940:Work Schedule Tolerance
D008550:Melatonin