Sleep duration or bedtime? Exploring the relationship between sleep habits and weight status and activity patterns.

STUDY OBJECTIVES

To assess the effects of early and late bedtimes and wake up times on use of time and weight status in Australian school-aged children.

DESIGN

Observational cross-sectional study involving use of time interviews and pedometers.

SETTING

Free-living Australian adolescents.

PARTICIPANTS

2200 9- to 16-year-olds from all states of Australia

INTERVENTIONS

NA.

MEASUREMENTS AND RESULTS

Bedtimes and wake times were adjusted for age and sex and classified as early or late using median splits. Adolescents were allocated into 4 sleep-wake pattern groups: Early-bed/Early-rise; Early-bed/Late-rise; Late-bed/Early-rise; Late-bed/Late-rise. The groups were compared for use of time (screen time, physical activity, and study-related time), sociodemographic characteristics, and weight status. Adolescents in the Late-bed/Late-rise category experienced 48 min/d more screen time and 27 min less moderate-to-vigorous physical activity (MVPA) (P<0.0001) than adolescents in the Early-bed/Early-rise category, in spite of similar sleep durations. Late-bed/Late-rise adolescents had a higher BMI z-score (0.66 vs. 0.45, P=0.0015). Late-bed/Late-rise adolescents were 1.47 times more likely to be overweight or obese than Early-bed/Early-rise adolescents, 2.16 times more likely to be obese, 1.77 times more likely to have low MVPA, and 2.92 times more likely to have high screen time. Late-bed/Late-rise adolescents were more likely to come from poorer households, to live in major cities, and have fewer siblings.

CONCLUSIONS

Late bedtimes and late wake up times are associated with an unfavorable activity and weight status profile, independent of age, sex, household income, geographical remoteness, and sleep duration.