ABSTRACT Alzheimer’s Disease and Related Dementias (ADRD) is an emerging epidemic in the United States; Black adults are particularly vulnerable. Without effective prevention strategies to target and optimize resilience to ADRD, 14 million U.S. adults will be living with ADRD by 2060, with an associated financial burden of $511 billion by 2040. While physical activity has been consistently identified as a modifiable, protective factor to prevent cognitive decline and delay age of dementia onset, the majority of this evidence is based on reported estimates of leisure-time moderate to vigorous intensity physical activity (MVPA), which provides an incomplete characterization of this complex behavioral phenotype. Adults spend approximately 95% of waking hours sedentary or in light intensity physical activity; however, there is limited research exploring the interrelations of these different intensity categories. Further, less than 10% of adults accumulate sufficient MVPA to meet recommendations, and this prevalence estimate is even lower in Black adults. Studies have also demonstrated that sleep disturbances, including short sleep duration, circadian rhythm disturbances, and sleep-disordered breathing may increase risk of cognitive impairment. However, this evidence is still emerging. Further, Black adults have a higher prevalence of disordered sleep compared to Whites. Despite the potential importance of both waking and sleep behaviors to optimize cognitive resilience, their impacts are often studied in isolation. To address these critical research gaps, we propose the Jackson Heart Study (JHS) 24-Hour Activity Cycle Study (Jackson 24H-ACT), a four-year ancillary study to the funded JHS Exam 4 (2020-22). The overall goal of the Jackson 24H-ACT Study is to quantify 24-hour activity cycle behaviors and estimate the independent and joint roles of waking (sedentary to vigorous intensity physical activity) and sleep (duration and quality characteristics) behaviors to optimize cognitive resilience. All participants attending Exam 4 and who are willing to concurrently wear an accelerometer and sleep actigraphy watch for seven consecutive days will be invited to participate. Exclusion criteria include: unable to ambulate or physician instruction not to be physically active. Jackson 24H-ACT ancillary study measures include: (1) accelerometry and (2) sleep actigraphy, which complement extant JHS data, and data that will be collected as part of Exam 4. To accomplish these goals, we propose to examine the independent and joint associations of 24-hour activity cycle behaviors with: (Aim 1) cognitive functioning; (Aim 2) subclinical markers of cerebrovascular disease (e.g. white matter hyperintensity volumes), white matter microstructural integrity, and neurodegeneration (e.g. hippocampal and total brain volumes) quantified from brain magnetic resonance (MRI) imaging; and (Aim 3) the prevalence of mild cognitive impairment (MCI) and dementia. Potential moderation...