AbstractObjectiveBeyond sleep duration, other facets of sleep such as variability and timing may be associated with obesity risk in youth. However, data are limited. Using a longitudinal design, this study tested whether multiple facets of sleep were associated with fat mass gain over 1 year.MethodsA convenience sample of non‐treatment‐seeking youth (age 8‐17 years) wore actigraphy monitors for 14 days. Average weekly sleep duration, within‐person sleep duration variability, weekend catch‐up sleep, bedtime and wake time shift, social jet lag, bedtime, wake time, and sleep midpoint were calculated. The association of each facet of baseline sleep with 1‐year fat mass, adjusting for baseline fat mass and height, was examined.ResultsA total of 137 youths (54.0% female; mean [SD], age 12.5 [2.6] years; 28.4% non‐Hispanic Black or African American; baseline fat mass = 15.3 [8.9] kg; 1‐year fat mass = 17.0 [10.0] kg; 28.5% with baseline overweight or obesity) were studied. Wake time (p = 0.03) and sleep midpoint (p = 0.02) were inversely associated with 1‐year fat mass, such that earlier wake time and midpoint were associated with higher 1‐year fat mass. No other facet of sleep was significantly associated with 1‐year fat mass (p > 0.09).ConclusionsUsing objective measures, youth with earlier wake times and sleep midpoints had greater gains in fat mass. Additional research is needed to determine whether sleep timing may be a modifiable target to prevent pediatric obesity.