Non-Wear Time and Presentation of Compositional 24-Hour Time-Use Analyses Influence Conclusions About Sleep and Body Mass Index in Children

Although 24-hour time-use data are increasingly being examined in relation to indices of health, consensus has yet to be reached about the best way to present estimates from compositional analyses. This analysis explored the impact of different presentations of results when assessing the relationship between 24-hour time-use and body mass index (BMI) z-score using compositional analysis of 5-day actigraphy data in 742 children. First it was found that reallocating non-wear time to day-time components only (sedentary behavior, light physical activity, and moderate-to-vigorous physical activity [MVPA]) before normalization to 24 hours provided stronger estimates with BMI z-score than simply removing non-wear time before normalization. Estimates for sleep time were substantially affected, where associations with BMI z-score nearly doubled (mean difference [95% CI] in BMI z-score for 10% longer sleep were −0.20 [−0.32, −0.08] compared to −0.11 [−0.23, 0.002]). Presenting estimates in terms of a greater number of minutes in a component, relative to all others, showed MVPA to be the strongest predictor of BMI z-score, while estimates in terms of the proportion of minutes showed sleep to be the strongest predictor. Both presentations have value. However, presentations in terms of one-to-one “substitutions” of time may need careful interpretation due to the uneven distribution of time in each component. In conclusion, when analyzing relationships between 24-hour time-use and health outcomes, non-wear time and presentation of estimates can impact final conclusions. As a result, the current understanding of the importance of sleep for child health may be underestimated.