Background: Cardiovascular Disease (CVD) is the most prevalent chronic disease in the aging population. Older women are at greatest risk for CVD, higher than their male counterparts. Disparities in the incidence and prevalence of CVD and CVD risk factors in older women, suggest that current prevention strategies are not effective for these populations. Research shows that post menopausal women experience difficulty adapting to age-related changes in sleep. Considering 1/3 of the day is spent sleeping, sleep duration is a potential modifiable CVD lifestyle risk factor worthy of greater exploration. Aging research to date has included a focus on the cardiometabolic associations of sleep duration, including large-scale epidemiological studies and meta-analyses concluding that inadequate sleep duration and poor sleep quality are associated with increased development, progression, and severity of CVD and CVD comorbidities. Research, however, has mostly employed self-reported sleep measures, focused on Caucasian populations and has not considered sleep as part of the 24 hour day. There is a need for research to examine the relationship between sleep duration and CVD risk in an older population of women, and to determine if insufficient sleep duration, short or long, is a CVD risk factor worthy of further examination. Methods: This dissertation leverages data from the unique “Objective Physical Activity and Cardiovascular Health” (OPACH) study among 6489 women, ages 63-99, recruited from a Women’s Health Initiative cohort. Women wore ActiGraph GT3X+ accelerometers on the hip for 24 hours per day and completed a daily sleep log over a 7-day period. In addition to accelerometer-measured 24-hour activity data, the OPACH study includes measures of physical functioning, lifestyle questionnaires, and clinical biomarkers. Participants were contacted yearly to provide updated medical history including self-report of CVD events with follow-up up to 5 years. Sleep data were scored according to a standard protocol using sleep logs. Chapter 1 assessed if sleep duration, measured with accelerometers, is associated with numerous cardiometabolic markers, including measures of insulin resistance, inflammation and body composition in older women. Chapter 2 was an examination of the relationship between sleep duration and the Reynolds Risk Score, a clinically relevant composite CVD risk score. In Chapter 3, the relationship between cardiovascular events over 3-5 years of follow-up and self-reported sleep duration and accelerometer-measured sleep duration was examined. Results: Chapter 1 found significant associations between sleep duration and markers of cardiometabolic health. Several of these relationships suggested the relationship was u-shaped, with short and long sleep associated with higher cardiometabolic values. Chapter 2 further explored this relationship and demonstrated that sleep duration is non-linearly associated with 10-year estimated CVD risk among older women. In Chapter 3, there was no significant increase in CVD risk for short or long sleep durations over the 5-year period, measured by self-report or accelerometer. Discussion: The results of this analysis demonstrate sleep duration is related to markers of cardiometabolic health and intermediate CVD risk, but that sleep duration does not independently predict incident CVD. These findings may be explained by the interdependence of 24-hour activities, including sleep duration, and their relationship to cardiometabolic health. While results do not support sleep duration as a risk factor for CVD, they do support sleep duration as a lifestyle behavior worth targeting for cardiometabolic risk reduction in older women. To better understand how sleep relates to cardiometabolic health, we must better understand the interdependence and interrelationships of activities throughout the 24-hour day.