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Objective: To determine the associations between screen media use and sleep throughout infancy (3-12 months)., Design: Prospective Nurture birth cohort., Setting: North Carolina, USA, 2013-2015., Participants: Women enrolled in their second to third trimester, completed a phone interview after birth, and completed home visits at 3, 6, 9 and 12 months post partum., Primary and Secondary Outcome Measures: Women reported the usual hours their infants slept during the day and night and their infants' usual use of five screen media activities at 3, 6, 9 and 12 months post partum. Adjusted mixed-effects regression analyses modelled the associations between infant screen time and sleep outcomes while disaggregating the between-infant and within-infant effects., Results: Among 558 mother-infant dyads, 374 (67.0%) infants were black and 304 (54.5%) households earned <$20 000 per year. Half (254, 50.2%) of the infants engaged with screens at 3 months of age, while 326 (72.9%) engaged at 12 months. The median value of the average daily screen time over the study period was 50 (IQR: 10-141) min. Infant screen time was inversely associated with night-time sleep duration only when considering between-infant effects (adjusted beta: -2.9; 95% CI -5.9 to 0.0; p=0.054 for log-transformed screen time). Effects were stronger for television+DVD viewing specifically (adjusted beta: -5.2; 95% CI -9.1 to -1.4; p<0.01 for log-transformed television+DVD time). For example, an infant who averaged 1 hour of television+DVD viewing over the study period slept, on average, 9.20 (95% CI 9.02 to 9.37) hours per night by 12 months compared with 9.60 (95% CI 9.41 to 9.80) hours per night for an infant with no screen time over the study period. There were no significant within-infant effects between screen time and night-time sleep, and screen time was not associated with daytime sleep or night-time awakenings., Conclusions: Screen time during infancy was inversely associated with night-time sleep duration; however, causal associations remain uncertain., Trial Registration Number: NCT01788644., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)