0415 Intra-Individual Variability in Sleep and Insomnia Severity in a Shift-Working Nursing Staff Sample

Abstract Introduction Greater intraindividual variability (IIV) in sleep parameters (e.g., total sleep time [TST], bedtime [BT], wake time [WT]) is associated with worse sleep outcomes, including more severe insomnia symptoms, maladaptive sleep-related cognitions, and greater fatigue. Greater IIV has also been linked to worse physical and mental health outcomes.. Shift workers comprise roughly 15% of the workforce and experience chronically elevated IIV in sleep imposed by their work schedule. Few studies have examined whether IIV predicts insomnia severity in this context of work-driven sleep variability, particularly using repeated, within-person measures. To address this gap, the current study examined associations between IIV in sleep parameters and insomnia severity. Methods Participants were shift-working nursing staff who took part in a parent study. Participants completed baseline questionnaires and 14 daily sleep diaries. From these data, we derived four nightly sleep variables, used as independent variables: 24-hour TST (sum of all sleep episodes within 24 hours), longest sleep episode TST, longest sleep episode BT and WT. Insomnia symptoms were assessed with the Insomnia Severity Index (ISI), used as a dependent variable. To model intraindividual variability (IIV), we used the Bayesian Variability Model implemented in the varian package (“V -> Y” design). Results Across models, greater IIV in sleep parameters was not associated with insomnia severity. Greater mean 24h TST was robustly associated with less insomnia severity (β = −2.14, 95% CrI [−4.15, −0.33], p_emp = .02). No other mean sleep parameter was associated with insomnia severity. Conclusion Contrary to prior research in non-shift workers, our analyses suggest that IIV in sleep parameters was not associated with ISI, whereas greater 24h TST was associated with less ISI. Though further investigation is needed, one possible explanation is that this discrepancy reflects differences in the degree of control individuals have over their daily schedules. Future research should also investigate whether increased TST, independent of variability, improves insomnia symptoms among shift workers. Support (if any)