The association between red blood cell distribution width‐to‐albumin ratio and risk of insomnia: A cross‐sectional analysis of National Health and Nutrition Examination Survey and Chinese clinical data

Abstract Background The red blood cell distribution width‐to‐albumin ratio (RAR) is a novel composite biomarker of inflammation and nutritional status, yet its association with insomnia and underlying mechanisms remain poorly understood. Objective To investigate the association between the RAR and insomnia in two independent cohorts, comprising a nationally representative U.S. population and a Chinese clinical population, and to explore the mediating roles of adiposity and metabolic indicators. Methods We analyzed 20,571 adults from National Health and Nutrition Examination Survey (NHANES) 2005–2014 and 270 participants from the Hangzhou Hospital of Traditional Chinese Medicine (HZ‐TCM) cohort in China. Insomnia was defined using validated questionnaires or clinical diagnoses. Logistic regression, restricted cubic spline analyses, two‐piecewise threshold analyses, and mediation analyses were performed. Results Threshold analysis identified RAR inflection points of 3.52 (NHANES) and 3.44 (HZ‐TCM). Below these thresholds, each unit increase in RAR was associated with a 2.86‐fold increase in insomnia odds in NHANES (OR = 2.86, 95% CI: 2.23–3.68) and a 27.74‐fold increase in the HZ‐TCM cohort (OR = 27.74, 95% CI: 9.11–84.45) (both p < 0.001), whereas no significant association was observed above the thresholds. Conclusions RAR is an independent and robust biomarker for insomnia risk, demonstrating a consistent nonlinear threshold association across both population‐based and clinical cohorts. Central obesity and insulin resistance appear to be important mediating pathways. As an easily accessible composite indicator, RAR holds potential clinical utility for insomnia risk assessment.