Rotating Shift Work and Menstrual Cycle Characteristics

Mar 3, 2011Epidemiology (Cambridge, Mass.)

How rotating shift work relates to menstrual cycle changes

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Abstract

Eight percent of participants reported working rotating night shifts for 1-9 months.

Women engaging in 20+ months of rotating shift work had a 23% higher risk of experiencing irregular menstrual cycles compared to those with 26-31 day cycles.
The likelihood of having a menstrual cycle shorter than 21 days was elevated by 27% among women with extensive rotating shift work.
A 49% increased risk of having menstrual cycles lasting 40 days or more was observed in women with 20+ months of rotating shift work.
A dose-response relationship was noted, with more months of rotating shift work correlating with higher instances of irregular cycles and longer cycle lengths.

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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.

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Rotating Shift Work and Menstrual Cycle Characteristics

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