PROJECT SUMMARY Toddlers spend half their day sleeping. While it is known that sleep supports learning in childhood, whether this function changes across early development is unknown. Recent theoretical work suggests that critical transitions in sleep's cognitive function occur at this age. Thus, longitudinal studies of sleep across toddlerhood are needed. The specific objective of the proposed research is to examine longitudinal changes in overnight sleep physiology and sleep's cognitive function in children 16-31 months of age. In doing so, we will address two proposed changes in sleep function in toddlerhood. First, critical changes in hippocampal development are suggested to change sleep's function for those <18 months relative to those >24 months. Second, models of brain metabolism suggest that the function of sleep, particularly REM sleep, changes from <28 months relative to those >30 months. To test these predicted turning points in sleep function, we will recruit infants who are 16 or 21 months and assess declarative memory before and after overnight sleep in 3 waves, separated by 5 months. Polysomnography will be used to understand the physiological mechanism underlying sleep benefits. Aim 1 is to examine whether memory benefits of overnight sleep change across toddlerhood. Specifically, we will consider whether there is a change in how beneficial sleep is relative to wake on memory outcomes across toddlerhood. We hypothesize that sleep will protect memories consistently across this age span but that memories will decay less over wake with development. Aim 2 is to examine whether the contribution of REM and non-REM sleep to memory change over sleep across toddlerhood. Specifically, we will examine predicted contributions of nREM oscillations and REM sleep to sleep-related memory changes across toddlerhood. Hypothesis 2 is that the change in memory over sleep is associated with coupling of sleep spindles and slow oscillations and, in particular, the phase coupling between these rhythms. This work will have clear theoretical significance for the fields of sleep and developmental sciences as we seek to test theories that have gained attention in those fields. Distinguishing the function of sleep across this age range will provide critical insight into mechanisms underlying sleep's memory benefit at any age. The work holds translational significance as well. Specifically, understanding sleep physiology and function at this age is critical for recognizing and understanding when these processes are impaired. Moreover, sleep's role in memory holds promise for interventions to improve learning delays or rehabilitation. As such a strong understanding of sleep's role in memory will eventually support such intervention approaches.