(HealthDay News) – There may be an increased risk of developing breast cancer among women who work night shifts.

To investigate the association between night shift work and the risk of breast cancer, Johnni Hansen, PhD, and Christina F. Lassen, MD, from the Danish Cancer Society in Copenhagen, identified 218 cases of breast cancer (1990–2003) from a cohort of female military employees, and selected 899 age-matched controls from the cohort by incidence density sampling. A questionnaire was used to gather information on shift work, sun exposure habits, and diurnal preferences.

The researchers found that, overall, there was an increased likelihood of breast cancer among women who had ever worked a night shift (adjusted odds ratio [OR], 1.4; 95% confidence interval, 0.9–2.1), compared with those never working night shifts. There was a significant increase in the relative risk of breast cancer with an increase in the number of years of night shift work (P=0.03) and with cumulative number of shifts worked (P=0.02). The risk was neutral for women who worked fewer than three night shifts per week. Among women with the highest tertile of cumulative exposure, the likelihood of breast cancer was significantly increased (OR, 2.3). The effect of night shift work was most pronounced in women with morning chronotype preference and intense night shifts (OR, 3.9). In addition, night-shift workers sunbathed more often than day-shift workers.

“The results indicate that frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of intense night shifts,” the authors write.

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