Nightshift work and irregular menstrual cycle: 8-year follow-up cohort study

Background Irregular menstruation is a major health problem among women, although its association with nightshift work remains controversial.Aims To study the association between nightshift work and irregular menstrual cycle among female workers and investigate any differences according to sleep quality, working hours or obesity.Methods This study included female workers who underwent health examinations from 2012 to 2019. Nightshift work, working hours, sleep quality and menstrual cycles were assessed using self-administered questionnaires. Irregular menstrual cycle was defined as self-reported irregular or >= 36 days. Adjusted odds ratios and 95% confidence intervals (CIs) were calculated by multivariable logistic regression; adjusted hazard ratios (95% CIs) for incident irregular menstrual cycle were calculated by Cox proportional hazard models with time-dependent analysis.Results The study participants were 87 147 in the cross-sectional study and 41 516 in the longitudinal study. After adjusting for all covariates in the cross-sectional analyses, the odds ratio for prevalent irregular menstrual cycle among female nightshift workers versus the reference was 1.26 (95% CI 1.2-1.33). In the cohort study, the adjusted hazard ratio for incident irregular menstrual cycle among nightshift workers was 1.95 (95% CI 1.61-2.35) in the period after 6 years. No significant differences were observed among subgroups stratified by sleep quality, working hours or obesity.Conclusions Nightshift work is associated with an increased risk of both prevalent and incident irregular menstrual cycle in female workers without significant interactions by sleep quality, working hours or obesity. Female workers suffer from irregular menstrual cycle, although its association with nightshift work remains controversial. We conducted a cross-sectional and longitudinal study of a large cohort undergoing routine health examinations at Kangbuk Samsung Hospital between 2012 and 2019. The analysis showed that nightshift work increased the risk of irregular menstrual cycle, and the results were consistent regardless of sleep quality, working hours, and obesity. It's worth noting that a long-term longitudinal study shows significant results.