Earlier bedtime at 1.5 years is associated with greater height gain at 3 years of age: a nationwide birth cohort study of the Japan environment and children's study.

Background: Previous studies have shown inconsistent associations between sleep duration and height gain in early childhood. The relationships of bedtime and nighttime awakenings with growth remain unclear. We investigated whether sleep characteristics at 1.5 years were associated with subsequent height gain. Methods: Forty-nine thousand two hundred eight singleton, term toddlers classified as appropriate for gestational age were selected from the Japan Environment and Children's Study. Nighttime sleep was defined as sleep between 20:00 and 07:59. Nighttime sleep duration, bedtime, and nighttime awakenings were categorized as long (≥10 h) or short (<10 h), early (20:00-21:59) or late (≥22:00), and infrequent (<3) or frequent (≥3), respectively. Linear mixed-effects models were used to examine associations with height and height standard deviation scores (SDS). Results: The early bedtime group showed greater monthly increases in height and height SDS than the late bedtime group, corresponding to 18-month differences of 0.13 cm ( Conclusion: Earlier bedtime at 1.5 years was associated with greater height gain, although the magnitude of the association was small. The associations of sleep duration with height gain varied between nighttime and 24-h sleep analyses. The observed association with earlier bedtime may have practical relevance because bedtime can be discussed during routine child health guidance; however, given the observational study design, these findings should not be interpreted as evidence of a causal effect.