Study Objectives: To provide reference data on sleep duration throughout childhood and explore the demographic characteristics associated with sleep. Design: Population-based prospective longitudinal birth-cohort study. Setting: South-West England, children born in 1991-1992 and followed since birth. Participants: Eleven thousand five hundred children with repeat measures of sleep from birth based on parent-reported questionnaires. Data on daytime and nighttime sleep duration and timings and night awakenings at 8 timepoints from age 6 months to 11 years. Results: Total sleep duration steadily fell from 13 hours and 12 minutes during infancy to 9 hours and 49 minutes at 11 years of age. Compared with earlier studies, the younger children in this cohort slept for a shorter period. The variation in sleep duration was very wide: from 10 to 17 hours in early infancy, narrowing to 8.5 to 11 hours at 11 years. Half of the children at preschool age woke at least once during the night, but frequent waking (>3 times) peaked in infancy (10% of all infants) and steadily declined in the preschool-aged years. Despite going to bed at the same time, girls slept consistently longer than boys (by 5-10 minutes). Children from low-income families went to bed later and woke up later, but there was little difference in total sleep duration. Children of younger mothers (35 years) slept persistently less. Children in larger families tended to go to bed later, as did the minority group of non-White children in the cohort. Conclusions: Given the wide natural variation of sleep in the childhood population, any recommendations on optimal sleep duration at any age must take into account considerable individual variability.