Measures Collected at Baseline
Demographic data were obtained from the parent, including age, sex, and race and ethnicity of the child; presence of any siblings; and maternal educational level (the index used in New Zealand; and all participating parents or guardians identified as mothers). Area-level deprivation for a family was measured with the 2018 New Zealand Index of Deprivation, an index based on the New Zealand Census of Population and Dwellings data that reflects the extent of material and social deprivation used to construct deciles from 1 (least deprived) to 10 (most deprived).Children wore an accelerometer (ActiGraph wGT3X-BT; ActiGraph LLC), which was set to 30 hz at initialization and downloaded with 15-second epochs, on their right hip 24 hours a day for 1 week to measure sleep timing (onset and offset), sleep duration (total sleep time), and sleep quality (number of awakenings, wake after sleep onset, and sleep efficiency). Actigraphy data were analyzed using a count-scaled algorithm developed in MATLAB (MathWorks Inc) that estimated sleep onset and offset as well as awakenings that were specific to each individual for each day.