The association between small-for-gestational-age infant status and childhood wake-up time and bedtime: a nationwide cohort study in Japan

Studies examining small-for-gestational-age (SGA) infants and sleep timing during childhood are scarce. We investigated the association between infant SGA status and sleep timing (wake-up time and bedtime) at age of 2.5 and 7 years using data from a Japanese birth cohort study conducted from 2001 ( n = 46,563). SGA was defined as birth weight below the 10th percentile for gestational age based on Japanese reference standards. The four sleep-timing outcomes were defined as wake-up time at or after 9:00 am or irregular and bedtime at or after 10:00 pm or irregular at 2.5 years, and wake-up time at or after 7:00 am or irregular and bedtime at or after 10:00 pm or irregular on school days at 7 years. These cut-off points were used as study-specific operational definitions based on the questionnaire response categories. Poisson regression with robust variance was used to estimate risk ratios (RRs) and 95% confidence intervals, adjusting for child and parental factors. In crude analyses, children born SGA had a slightly higher RR for bedtime at or after 10:00 pm or irregular at 2.5 years; however, this association was not significant after adjustment. No significant association was observed for the other outcomes. Sensitivity and stratified analyses yielded similar results.