PROJECT SUMMARY Although physical activity, sedentary behavior, and sleep (i.e., 24-hr behaviors) have been associated with cognitive and brain outcomes in older children and adults, studies are limited in early childhood and typically examine these behaviors separately. Given that early childhood serves as an important time for brain and cognitive development and when healthy habits (i.e., low sedentary time, high physical activity, and sufficient sleep) are formed, the goal of this research is to examine the relations between 24-hr behaviors, cognition, and brain structure (i.e., hippocampal volume, as this structure is a key brain region to learning and memory) in early childhood. The aims of this proposal are to determine if 24-hr behaviors are associated with 1) cognitive performance and 2) hippocampal volume in early childhood. Data from two ongoing studies examining the benefits of napping on memory in early childhood (i.e., children ages 33 to 71 months) will be used: a clinical trial (NIH R01 HL111695) with one measurement period and a longitudinal clinical trial (NIH R21 HD094758/NSF 1749280) with three measurement periods over one year. To address the aims of this project, cross-sectional and longitudinal analyses from various subgroups of participants of the two studies will be conducted. Measurements will include time spent in each of the 24-hr behaviors from 16-days of actigraphy (i.e., accelerometry via a wrist monitor), cognitive performance from multiple assessments, and hippocampal volume from magnetic resonance imaging. The aims of this proposal have public health significance in that they will identify potential windows of opportunity to intervene on health behaviors, learning, and cognitive function at a critical developmental period of the lifespan. The findings can inform future intervention studies, family practices, early childhood education policies, and comprehensive guidelines for a 24-hr cycle. Collectively, the proposed development plan incorporates activities for training in developmental science, sleep, neurocognitive measurement, statistical analyses, and scientific writing. Successful completion of this proposed training plan will result in several submissions of first-author manuscripts to peer-reviewed journals and an essential research foundation to support a competitive early career funding proposal. The fellowship will provide protected time and unique cross-disciplinary mentorship for successful completion of research and training goals.