Background: Shift workers who experience sleep disturbances and exposure to light at night could be at increased risk for alterations in physiologic functions that are circadian in nature.
Methods: We investigated rotating shift work and menstrual cycle patterns in the Nurses' Health Study II using cross-sectional data collected in 1993 from 71,077 nurses aged 28–45 years who were having menstrual periods and were not using oral contraceptives. Log-binomial regression was used to estimate relative risks (RRs) and 95% confidence intervals (CIs).
Results: Eight percent of participants reported working rotating night shifts for 1–9 months, 4% for 10–19 months, and 7% for 20+ months during the previous 2 years. Irregular cycles (>7 days variability) were reported by 10% of participants. Seventy percent of women reported menstrual cycles of 26–31 days, 1% less than 21 days, 16% 21–25 days, 11% 32–39 days, and 1% 40+ days. Women with 20+ months of rotating shift work were more likely to have irregular cycles (adjusted RR = 1.23 [CI = 1.14–1.33]); they were also more likely to have cycle length <21 days (1.27 [0.99–1.62]) or 40+ days (1.49 [1.19–1.87]) (both compared with 26–31 days). For irregular patterns and for 40+ day cycles, there was evidence of a dose response with increasing months of rotating shift work. Moderately short (21–25 days) or long (32–39 days) cycle lengths were not associated with rotating shift work.
Conclusions: Shift work was modestly associated with menstrual function, with possible implications for fertility and other cycle-related aspects of women's health.
From the aNational Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Cincinnati, OH; bChanning Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA; Departments of cEpidemiology and dBiostatistics, Harvard School of Public Health, Boston, MA; and eConnors Center for Women's Health and Gender Biology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA.
Submitted 22 March 2010; accepted 8 December 2010; posted 1 March 2011.
Supported (partially) by Centers for Disease Control/National Institute for Occupational Safety and Health (contract 200–2001–08007).
The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
Correspondence: Christina C. Lawson, NIOSH, CDC, 4676 Columbia Parkway, R-15, Cincinnati, OH 45226. E-mail: firstname.lastname@example.org.