AimSleep duration and pattern have important implications for children's health. This study aims to investigate nocturnal sleep duration, sleep pattern and their relationships with outdoor play and screen time among children aged 2 to five years. MethodsThe study used data from the Healthy Beginnings Trial undertaken in Sydney, Australia. Data on children's sleep, outdoor playtime and screen time were reported by mothers via face-to-face interviews when children were 2, 3.5 and five years old. Both cross-sectional and longitudinal analyses were conducted. ResultsAt age 2, 3.5 and five years, 497, 415 and 369 mother-child dyads participated. Significantly, there was an overall increase in children's nocturnal sleep duration, sleep latency and an earlier bedtime, and there was a decrease in the proportion of children who woke at night over time. Each additional hour of screen time was associated with three-minute (95% CI 0.6-5) shorter sleep, 1.6-minute (95% CI 0.59-2.63) longer sleep latency, four-minute (95% CI 1.8-6.0) later bedtime and less likely sleeping 10hours per night with adjusted odds ratio 0.88 (95% CI 0.77-1.00), after controlling for mothers' demographics. ConclusionAmong young children, screen time and outdoor playtime were associated with sleep duration and pattern. Reducing screen time and increasing outdoor playtime might help improving children's sleep.