BACKGROUND: Sleep disturbances and cardiovascular disease are common and often co-occur in middle-aged and older adults, but less is known about associations of sleep with cardiorespiratory fitness (CRF) and energy efficiency in these populations. We examined cross-sectional associations of accelerometer-derived sleep metrics with CRF, walking energetics, and resting metabolic rate (RMR), and explored whether associations were moderated by age, sex, and race. METHODS: We studied 263 participants from the Baltimore Longitudinal Study of Aging (mean age 72.7 ± 10.1 years, 53.6% women). Predictors included total sleep time (TST), sleep efficiency (SE), sleep onset latency, wake after sleep onset, and average wake bout length (WBL). Outcomes included measures of CRF (ie, maximal oxygen consumption [VO2peak]) and energetic efficiency (ie, energetic cost of walking and RMR). RESULTS: After adjusting for demographics, comorbidities, and self-reported physical activity, longer WBL was associated with lower VO2peak (B= -1.01 ml/kg/min, P < .01) and higher RMR (B = 43.25 kcal, P < .05), lower SE was associated with lower VO2peak (B = 1.07 ml/kg/min, P < .01), and shorter TST was associated with lower VO2peak (B = 0.33 ml/kg/min, P < .05). Higher SE was associated with lower RMR among middle-aged adults but not older adults (interaction P-value < 0.05). CONCLUSION: Shorter TST, longer WBL, and lower SE are associated with poorer CRF and energetic efficiency among middle-aged and older adults. Longitudinal studies are needed to understand the temporality of these associations and potential targets for interventions in these populations.