OBJECTIVES: Sleep is an emerging area of concern in children with juvenile idiopathic arthritis (JIA). Research shows the presence of poor sleep quality and related adverse outcomes in pediatric pain populations, including JIA, but few studies have examined the prospective patterns of association between sleep and associated outcomes. This prospective study evaluated the direction and magnitude of associations between subjective sleep characteristics (sleep quality, difficulty initiating sleep, and sleep duration), pain intensity, and functional limitations in children with JIA. We hypothesized that pain intensity would partially mediate the relationship between sleep and functional limitations. METHODS: Children and adolescents with JIA (n=59; age range, 8 to 18 y) recruited during clinic visits, completed smartphone-based diaries for 1 month. Subjective sleep characteristics were reported each morning; pain and functioning were assessed 3 times daily. RESULTS: As hypothesized, the associations between sleep quality and functional limitations and between difficulty initiating sleep and functional limitations were partially mediated by pain intensity, at any given moment (z=-3.27, P=0.001, z=2.70, P<0.05). Mediation was not detected in a model testing the association between sleep duration, pain intensity, and functional limitations (z=-0.58, P=0.56). DISCUSSION: Results suggest that sleep is integral to understanding the momentary association between pain intensity and functioning in children with JIA.