Physical activity is associated with improved sleep quality and duration in the general population, but its effect on sleep in postpartum women is unknown. We examined cross-sectional and longitudinal associations between hours/week of self-reported domain-specific and overall moderate to vigorous physical activity (MVPA) and sleep quality and duration at 3- and 12-months postpartum among a cohort of 530 women in the Pregnancy, Infection, and Nutrition Postpartum Study. MVPA was not associated with sleep quality or duration at 3-months postpartum. At 12-months postpartum, a 1 h/week increase in recreational MVPA was associated with higher odds of good (vs. poor) sleep quality (odds ratio, OR 1.14; 95 % confidence interval, CI, 1.03–1.27) and a 1 h/week increase in child/adult care MVPA was associated with lower odds of good (vs. poor) sleep quality (OR = 0.93; 95 % CI 0.88–0.99). A 1 h/week increase in child/adult care MVPA (OR 1.08, 95 % CI 1.00–1.16) was associated with higher odds of long sleep duration and 1 h/week increases in indoor household (OR 1.09, 95 % CI 1.01–1.18) and overall MVPA (OR 1.04, 95 % CI 1.01–1.07) were associated with higher odds of short (vs. normal) sleep duration. Comparing 3-months postpartum to 12-months postpartum, increased work MVPA was associated with good sleep quality (OR 2.40, 95 % CI 1.12–5.15) and increased indoor household MVPA was associated with short sleep duration (OR 1.85, 95 % CI 1.05–3.27) as measured at 12-months postpartum. Selected domains of MVPA and their longitudinal increases were associated with sleep quality and duration at 12-months postpartum. Additional research is needed to elucidate whether physical activity can improve postpartum sleep.