We examined the association between sleep behavior and cognitive functioning in 60 healthy children between 7 and 11 years of age under nonexperimental conditions. Intellectual abilities were assessed by the Wechsler Intelligence Scale for Children (4th edition) and sleep variables by questionnaires, actigraphy, and sleep diaries. Correlation analysis revealed a negative association between sleep duration on weekends and measures of intelligence (full-scale IQ, r = -.29; fluid IQ, r = -.36). The regression coefficient for sleep duration on weekends was -6.11 (SE = 2.09), indicating an increase of 6.11 points on fluid IQ scores for each hour of shorter sleep duration. Attention measures did not correlate with cognitive or sleep variables. Daytime sleepiness as a potential moderator of the relationship between sleep duration and cognitive performance was not related to cognitive or sleep variables. We conclude that children with higher daytime cognitive efficiency (reflected by higher intelligence scores) show increased nighttime efficiency (reflected by shorter sleep duration). In the light of the neural efficiency hypothesis, the current results argue for an extension of the original theory-referring not only to daytime but also to nighttime behavior.