Association between unpredictable work schedules and insomnia symptoms among wage workers in South Korea: the sixth Korean Working Conditions Survey

BACKGROUND: Unpredictable work schedules (UWSs), which are characterized by last-minute schedule changes, on-call shifts, and short advance notice, have become increasingly common and have been linked to disrupted circadian rhythms, impaired sleep homeostasis, and poor health. However, evidence from South Korea, where long hours and low schedule autonomy are prevalent, remains scarce. This study examined the association between UWS and insomnia symptoms among wage workers in South Korea. METHODS: Data from 20,534 wage workers aged 20-64 years were obtained from the 2020 Korean Working Conditions Survey. UWS was defined using three items: sudden callbacks, advance notice of schedule changes, and control over working hours. Insomnia symptoms were measured using the minimal insomnia symptom scale, with scores ≥ 6 indicating insomnia. Weighted multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for insomnia by UWS status after adjusting for demographic, socioeconomic, occupational, and psychosocial factors. Sensitivity analyses applied alternative UWS definitions (sudden recall, combined, strict, and lenient definitions). RESULTS: The prevalence of insomnia symptoms was 16.10% (157/975) in UWS and 7.25% (1418/19559) in non-UWS (p < 0.001). In the unweighted analysis, UWS was associated with higher odds of insomnia symptoms in the crude model (OR 2.46, 95% CI 2.05-2.94). Moreover, the association remained significant after adjustment for demographic and work-related covariates (Model 1: OR 2.15, 95% CI 1.78-2.59; Model 2: OR 1.64, 95% CI 1.34-2.01). Sensitivity analysis using alternative UWS definitions showed a consistent, statistically significant association in both crude and adjusted models. In the weighted analysis, the association remained significant after full adjustment (OR 2.30, 95% CI 1.74-3.05). CONCLUSIONS: UWS was found to be associated with higher odds of insomnia symptoms among Korean wage workers. Improving the predictability of schedules and ensuring the provision of advance notices may help improve sleep health. Longitudinal studies are needed to clarify causal relationships and to further investigate potential underlying pathways.