Adequate physical activity is associated with improved sleep in adults. Such associations, however, vary greatly across studies in pediatric populations, with no studies involving infants found in the literature. This prospective observational study was designed, therefore, to examine the association between physical activity and sleep patterns in infants. A total of 183 healthy infants aged 6 months wore an actigraph for 7 days to measure physical activity and sleep. Parents and caregivers completed a sleep-activity diary over the same period documenting the different types of infant activities. Daily mean time spent in screen-time-or-limited physical activity, including screen-based (e.g., watching television or other electronic devices) and nonscreen based activity (e.g., quiet play or restricted infant movement when carried by caregivers or seated in high chairs, swings, or bouncer seats), was 6.68 hr (SD = 1.99), which represented 47.50% of daytime waking hours (SD = 13.73). We found that 65 (35.5%) infants engaged in some screen time during the study, with 10 (5.5%) infants having an average daily screen time >30 min. In our multivariate linear regression model, more hours of screen-time-or-limited physical activity per day were significantly associated with a decrease in total daily 24 hr sleep duration (p < 0.01).Findings from our study suggest that reducing screen-time-or-limited physical activity might be an approach for promoting adequate sleep and lengthening infant daily sleep duration. K E Y W O R D Sactigraphy, infant, physical activity, sleep