ObjectivesTo establish unconditional reference centiles for sleep parameters in infants 4–16 weeks of age.Design and settingSecondary data analysis of sleep parameters recorded at 4–16 weeks of age in a longitudinal randomised controlled trial (RCT) (BabySMART).PatientsHealthy term infants assigned to the non-intervention arm of the RCT.Main outcome measuresInfants’ sleep duration was recorded by parents/guardians daily, from week 2–16 of age using a sleep diary. Reference centiles for total, daytime, night-time and longest sleep episode duration were estimated using multilevel modelling.ResultsOne hundred and six infants, mean (SD) gestational age of 39.9 (1.2) weeks and mean (SD) birth weight of 3.6 (0.5) kg had sleep recorded contributing 1264 measurements for each sleep parameter. Between 4 and 16 weeks of age total sleep duration in a 24-hour period, night-time sleep duration in a 12-hour period and infant’s longest sleep episode duration increased, while daytime sleep duration in a 12-hour period decreased.ConclusionsReference centiles up to 4 months of age in infants highlight the gradual decrease in daytime sleep and large increases in night-time sleep, which occur in tandem with increasing lengths of sleep episodes. These reference centiles provide useful sleep values for infant sleep trajectory occurring in early life and may be helpful for parents and clinicians.Trial registration numberNCT03381027.