Physical activity (PA), sedentary behavior (SB), and sleep all impact cognitive and physical health in aging. Most research to date has focused on how time spent in each of these behaviors individually impacts health outcomes. However, interactions of PA, SB, and sleep likely account in part for the tremendous heterogeneity in aging-related health outcomes, and their optimal balance across the 24-hour day in supporting healthy aging is not known. This proposal capitalizes upon unique resources available through the Adult Changes in Thought (ACT) U19 Program to explore the inter-relatedness of PA, SB, sleep, and longitudinal trajectories of cognitive and physical health. The 24-hour activity cycle paradigm as a model for determining longitudinal inter- relatedness of PA, SB, and sleep on health outcomes is innovative, as is our proposed collection of device- based data to complement self-reported outcomes. This project proposes to use a number of innovative statistical approaches to analyze the complex relationships between the 24-hour activity cycle and trajectories of cognition and physical functioning. Scientific rigor is supported by objective monitoring of 24-hour behaviors and builds upon more than two decades of ACT data in a well-characterized clinical population. Measures of cognitive and physical function will include Cognitive Abilities Screening Instrument item response theory scores, specialized tests of executive functioning and verbal fluency, self-reported activities of daily living, gait speed, rising from a chair, and grip strength. For Aim 1, we will test bidirectional associations relating cognitive and physical function to the 24-hour activity cycle using device-assessed measures of PA and SB, and self- reported sleep. We hypothesize that (a) historically better cognitive and physical function will be independently associated with higher levels of PA, lower SB, and better sleep in ACT participants, and (b) more PA, less SB, and better sleep will be independently associated with better future maintenance of cognitive and physical function. For Aim 2, we hypothesize that (a) historically better cognitive and physical function will be independently associated with better device-assessed sleep quality and circadian rhythms in ACT participants, and (b) better device-assessed sleep quality and circadian rhythms will be independently associated with better future maintenance of cognitive and physical function. For both Aims, we will construct combined 24- hour activity cycle profiles (based on different balances of PA, SB, and sleep) and explore how profiles are associated with historical trajectories of and future changes in cognitive and physical function. Collaboration with other Cores will provide new data about how Life Course and Clinical Core variables influence associations, as well as new information about relationships between the 24-hour activity cycle and novel neuroimaging data. Ultimately, our results will provide much ...