ObjectiveTo examine associations between sleep and flourishing among children ages 0-5 years in the United States and whether these differ by age, developmental needs, and family resilience. Study Design and MethodsCross-sectional data from the 2020-2021 National Survey of Children’s Health (N = 31,095) were used with survey-weighted logistic regression to explore associations between insufficient sleep (defined as not meeting age-recommended daily sleep guidelines: 12-16 hours for 4- to 12-month-olds, 11-14 hours for 1- to 2-year-olds, and 10-13 hours for 3- to 5-year-olds) and flourishing (using four markers combined and categorized into two groups). Tests of effect measure modification (EMM) were performed on the multiplicative and additive scales. ResultsWeak but notable evidence was found that children with insufficient sleep had decreased odds of flourishing (aOR = 0.76; 95% CI: 0.60, 1.00). No evidence of EMM by child age or family resilience was found. However, the sleep- flourishing association differed significantly by children’s developmental needs, suggesting that the combined effect of sleep and developmental needs impact flourishing more than either factor alone. ConclusionsApproximately 38% of children ages 0-5 years in the United States are estimated to have insufficient sleep. This study provides evidence that insufficient sleep is associated with decreased flourishing among children with special health care needs (CSHCN). Future ImplicationsIncreasing sleep interventions among children under five is warranted among children with special health care needs. The association between sleep and flourishing within specific CSHCN categories, including emotional, behavioral, or developmental (EBD) criteria, should be explored to optimize sleep policies.