Purpose: This study aimed to investigate whether insomnia among newly-employed shift-working nurses is associated with menstrual cycle irregularity.
Methods: We followed 287 nurses employed between 2015 and 2016 for 12 months. An Insomnia Severity Index (ISI) was completed to determine levels of insomnia 6 months after beginning work. At baseline and at 12-month follow-up, menstrual cycle information was obtained using clinical interviews. We analyzed baseline data separately by prevalence (participants with menstrual irregularity, n = 287) and incidence (participants with no reported menstrual irregularity, n = 238).
Results: In a multivariate logistic regression analysis, including age, body mass index, and physical activity, insomnia was associated with a 2.05-fold increase in the odds of newly developing menstrual cycle irregularity compared with not having insomnia [odds ratio (OR) 2.05, 95% confidence interval (CI) 1.12-3.77]. Insomnia was associated with a 3.05-fold increase in the prevalence of menstrual cycle irregularity compared with not having insomnia (OR 3.05, 95% CI 1.81-5.13). As the ISI score increased, both the incidence and prevalence odds of menstrual cycle irregularity tended to increase.
Conclusions: Insomnia can induce menstrual cycle irregularity among nurses working shifts, and insomnia may have an important role in the pathway from shift work to menstrual dysfunction, aside from the possible effects of circadian rhythm disruption.
Keywords: Circadian rhythm; Infertility; Menstrual cycle irregularity; Menstruation; Shift worker; Sleep disturbance.