1 Department of Human Kinetics, Saint Francis Xavier University, Antigonish, Canada.
2 Department of Pediatrics, Research Center, Sainte-Justine University Hospital Research Center and Université de Montréal, Montréal, Canada.
3 Department of Family Medicine, McGill University, Montréal, Canada.
4 Department of Mathematics and Statistics, Concordia University, Montréal, Canada.
5 Department of Kinesiology, Université Laval, Québec, Canada.
6 School of Kinesiology and Physical Activity Sciences, Université de Montréal, Montréal, Canada.

Recommendations for physical activity (PA) typically focus on frequency, intensity, duration, and type, but timing (chrono-exercise) is also important. The objective of this study is to describe when children are active on school and weekend days and explore PA timing across sex and body mass index (BMI) categories. 359 children (53% male), aged 9.6 (0.9) y, were categorized as normal weight (=-1 standard deviations (SD) and <1SD; n = 193), overweight (=1SD and <2SD; n = 80), or obese (=2SD; n = 86) using WHO BMIz. Moderate-to-vigorous PA (MVPA) was assessed using ActiGraph LS-7164. The results are described as Mean(SD). ANOVA evaluated MVPA across sexes and BMI categories. Normal weight boys were more active than boys with obesity on school (?20.33 min; p < 0.001) and weekend days (?15.04 min; p < 0.05). On school days, significant differences existed between 9:00 h-11:00 h and 12:00 h-14:00 h (p < 0.017), while on weekends, smaller differences existed throughout the day. Girls' MVPA was similar across BMI categories, on all days (p > 0.05). On school days, 12:00 h-13:00 h represented the most active hour for all participants (~14% total daily MVPA). Peak weekend MVPA was distributed across multiple hours. Differences in MVPA timing emerged on school-days and weekends. Timing may be important when examining the nuances of MVPA in relation to sex and bodyweight in children.