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Study Objectives Pre- and early adolescence are believed to constitute periods of important age-related changes in sleep. However, much of the research on these presumed developmental changes has used cross-sectional data or subjective measures of sleep, limiting the quality of the evidence. Additionally, little is known about the development of certain features of the sleep-wake cycle pertaining to regularity (e.g., weekend-weekday differences and intra-individual variability) or circadian rhythms (e.g., sleep midpoint). Methods This study examined the sleep trajectories of 128 typically developing youth (69 girls) from ages 8-12 years on four sleep characteristics: sleep onset, sleep offset, total sleep time (TST), and sleep midpoint. For each of these characteristics, actigraphy-derived estimates of typical (i.e., mean) sleep and sleep regularity were obtained at each time point. Multilevel growth curves were modeled. Results Overall, the sleepwake cycle significantly changed between 8 and 12 years. Mean sleep onset, offset and midpoint exhibited an ascending curvilinear growth pattern that shifted later with age, while mean TST decreased linearly. Weekend-weekday differences (social jetlag) for sleep offset and midpoint became more pronounced each year. Weekday TST was longer than weekend TST, though this difference became smaller over time. Finally, intra-individual variability increased over time for all sleep characteristics, with variability in TST ascending curvilinearly. Important between-person and sex differences were also observed. Conclusion This study reveals the marked changes that occur in the sleep of typically developing pre- and early adolescents. We discuss the potential implications of these trajectories.