The primary goal of this study was to determine whether sleep duration moderated the relations of two dimensions of children's temperament, shyness and negative emotion, to academic achievement. In the autumn, parents and teachers reported on kindergarteners' and first graders' (N = 103) shyness and negative emotion and research assistants observed negative emotion in the classroom. In the spring, children wore actigraphs that measured their sleep for five consecutive school nights, and they completed the Woodcock Johnson-III standardized tests of achievement. Interactions between temperament and sleep duration predicting academic achievement were computed. Interactions of sleep duration with parent-reported shyness, teacher-reported negative emotion, and observed negative emotion indicated that the negative relations of shyness or negative emotion to academic achievement were strongest when children slept less. Results suggest that sleep duration may be an important bio-regulatory factor to consider in young children's early academic achievement.