Association between systemic immune-inflammation index and sleep disorder among US adults: A cross-sectional study based on NHANES 2005 to 2020

The relationship between the Systemic Immune-Inflammation Index (SII) and sleep disorders remains poorly characterized. This cross-sectional study investigated this association utilizing data from the National Health and Nutrition Examination Survey spanning 2005 to 2020. A total of 42,150 participants were included, of whom 25.34% reported sleep disorders. The association between SII and sleep disorders was evaluated using multivariable logistic regression models and restricted cubic spline analysis. In unadjusted models, a significant positive association was observed; compared with the lowest SII quartile (Q1), the highest quartile (Q4) had 20.32% higher odds of sleep disorders (OR = 1.2032; 95% CI: 1.1309-1.2801; P < .0001). After adjustment for age, sex, and race, the association persisted (OR = 1.1118, 95% CI: 1.0425-1.1858, P = .0013). In more fully adjusted models (including body mass index, smoking, education, poverty-income ratio, diabetes, cardiovascular disease, and hypertension), the linear association was not statistically significant. Restricted cubic spline analyses showed a U-shaped association. Subgroup analyses indicated statistical effect modification by sex and by hypertension status. Overall, higher SII was associated with increased odds of reporting sleep disorders in partially adjusted models; however, in the fully adjusted models the association was not statistically significant (P > .05), suggesting that it may largely reflect shared correlates rather than an independent effect. Future longitudinal studies are warranted to clarify temporal relationships and to assess the incremental clinical utility of SII in sleep health.