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This study aimed to elucidate the association between sleep and academic performance using standardized academic assessment in a large and representative sample of school children and adolescents in Hong Kong. This school-based cross-sectional study was conducted in 2016. Students completed territory-wide standardized tests in Chinese, English, and Mathematics and a set of questionnaires covering sleep, academic anxiety and motivation. Parents provided additional information on socioeconomic status and children's study behaviors. Weekday proxy sleep duration was reflected by time-in-bed, the difference between bedtime and wakeup time. The study included 4262 Grade 3 (G.3) (mean age [SD], 9.2 [0.6] years; girls: 49.7%) and 3297 G.9 students (mean age [SD], 15.3 [0.74]; girls: 57.5%) from 77 schools. Apart from showing a general insufficiency of students' sleep in this metropolitan city, there was a significant quadratic relationship (G.3: β = −0.05, p <.001; G.9: β = −0.03, p <.01), indicating that students with an optimal level of sleep (9.5 h and 8.5 h in G.3 and G.9, respectively) tend to have better academic performance. The significant association remained after controlling for socioeconomic and study-related variables in that sleeping too little or too much was associated with poor academic performance. This is the first study to explore the curvilinear association between sleep duration and academic performance by using standardized test and simultaneously examining learning-related controls with a large representative sample in Hong Kong. The findings suggested that there is an inverted U-shaped relationship between sleep duration and academic performance in both school-aged children and adolescents. It is recommended that systematic sleep education and intervention is necessary to encourage the development of optimal sleep pattern, which might have beneficial effect on academic performance for students at both primary and secondary level. • There is a curvilinear relationship between sleep duration and academic performance in large representative samples. • The inverted U-shaped relationship between sleep duration and academic performance remained after controlling academic related confounding factors. • Systematic sleep education and intervention is necessary to encourage the development of optimal sleep pattern, which might have beneficial effect on academic performance. [ABSTRACT FROM AUTHOR]