Clear associations of sleep, cognitive performance, and behavioral problems have been demonstrated in meta-analyses of studies in adults. This meta-analysis is the first to systematically summarize all relevant studies reporting on sleep, cognition, and behavioral problems in healthy school-age children (5-12 years old) and incorporates 86 studies on 35,936 children. Sleep duration shows a significant positive relation with cognitive performance (r = 0.08, confidence interval [CI] [0.06, 0.10]). Subsequent analyses on cognitive subdomains indicate specific associations of sleep duration with executive functioning (r = 0.07, CI [0.02, 0.13]), with performance on tasks that address multiple cognitive domains (r = 0.10, CI = [0.05, 0.16]), and with school performance (r = 0.09, CI [0.06, 0.12]), but not with intelligence. Quite unlike typical findings in adults, sleep duration was not associated with sustained attention and memory. Methodological issues and brain developmental immaturities are proposed to underlie the marked differences. Shorter sleep duration is associated with more behavioral problems (r = 0.09, CI [0.07, 0.11]). Subsequent analyses on subdomains of behavioral problems showed that the relation holds for both internalizing (r = 0.09, CI [0.06, 0.12]) and externalizing behavioral problems (r = 0.08, CI [0.06, 0.11]). Ancillary moderator analyses identified practices recommended to increase sensitivity of assessments and designs in future studies. In practical terms, the findings suggest that insufficient sleep in children is associated with deficits in higher-order and complex cognitive functions and an increase in behavioral problems. This is particularly relevant given society's tendency towards sleep curtailment. (Contains 5 figures and 5 tables.)