It is often assumed that adequate sleep is a key ingredient of children's school success. Research to date, however, suggests modest associations between child sleep and academic achievement. Adopting a developmental perspective, this report investigates the associations between age‐related changes in sleep across the preschool period and academic achievement at school entry. Sleep was assessed by actigraphy at ages 2, 3 and 4 among 128 children from mostly White middle‐class families, and their performance in reading and mathematics was tested in Grade 1. The results revealed that children whose sleep duration decreased more rapidly across the preschool period showed better performance in both reading and mathematics. These results suggest that age‐related developments may be a key characteristic of sleep in the preschool years.