PROJECT SUMMARY Sleep is critical for infant physical and cognitive development and yet understanding sleep time and timing is a source of stress to parents and families. Thus a better understanding of when infants sleep, specifically the transition from two to one naps, will support infant cognitive development and contribute to guidance for families. The objective of the proposed research is to examine, longitudinally, the role of multiple sleep bouts in memory consolidation across the triphasic to biphasic sleep transition in infancy. The overarching hypothesis is that naps maintain their usefulness across these transitions, but that multiple naps become less essential to the preservation of memories as infants naturally transition to one nap per day. More specifically, it is predicted that memories can be held longer without interference as the child develops across this age range, making the morning nap less essential to memory while the afternoon nap remains essential. Participants will be 100 infants, who will complete 2 sessions, a sleep and a wake session, every three months (9, 12, and, 15 months). A deferred imitation task will be used to assess nap-related changes in memory consolidation. Actigraphy will be used to objectively assess nap habituality. Polysomnography will be used to understand the physiological mechanism underlying memory benefits and it will also provide a proxy for brain development. Collectively, the work will (1) assess memory consolidation over morning and afternoon nap intervals across the triphasic to biphasic sleep transition in infants; (2) examine the mechanism supporting declarative memory consolidation in infancy; and (3) establish the developmental trajectory of key sleep physiology features within naps across this developmental age range. An exploratory aim will examine whether changes in estimated brain development predict the triphasic to biphasic sleep transition. The outcomes have theoretical significance. These results will be significant for the field of sleep research, providing critical insight into development of sleep regulation processes and longitudinal changes in sleep patterns. Additionally, results will be informative to developmental scientists, suggesting that the timing of studies will contribute to performance. The outcomes also have translational significance. A better understanding of nap transitions will inform pediatricians and parents as they make recommendations and decisions about infant sleep and allow for identification of infants with abnormal sleep development trajectories.