Abstract The present study examined the associations between physical characteristics of neighborhoods and sleep health outcomes and assessed the mediating role of physical activity on these associations. A longitudinal study (PHRESH Zzz, n=1,051) was conducted in two low-income, predominately African-American neighborhoods with repeated measures of neighborhood characteristics and sleep health outcomes from 2013 to 2018. Built environment measures of walkability, urban design, and physical neighborhood disorder were captured from systematic field observations. Sleep health outcomes included insufficient sleep, sleep duration, wakefulness after sleep onset (WASO), and sleep efficiency measured from 7-day actigraphy data. G-computations based on structural nested mean models were used to examine the total effects of each built environment feature and causal mediation analyses were used to evaluate direct and indirect effects through physical activity. Urban design features were associated with decreased WASO (β: -1.26, 95% confidence interval [-4.31, -0.33]). Neighborhood disorder (β: -0.46, CI [-0.86, -0.07]) and crime rate (β: -0.54, CI [-0.93, -0.08]) were negatively associated with sleep efficiency. Neighborhood walkability was not associated with sleep outcomes. We did not find a strong and consistent mediating role of physical activity. Interventions to improve sleep should target modifiable factors, including urban design and neighborhood disorder.