Too tired to move? Temporal and longitudinal associations of social jetlag with physical activity and sedentary behavior in preschool children

Global public health guidelines recently moved from an isolated to an integrated approach for movement behaviors (sleep, physical activity and sedentary behavior) across a 24-hour period, recognizing that the whole day matters for optimal health for preschool children (3-5 years). In Hong Kong, much effort has been devoted to promoting healthy eating and active living for 3- to 6-year-olds since 2012. However, sleep has been overlooked in such campaign despite that all the movement behaviors interact and influence each other. Of particular concern is that local preschoolers have demonstrated the latest bedtimes compared with their counterparts in other countries.

Studies examining interrelationships between sleep and daytime activity behaviors have provided mixed results and have primarily focused on sleep duration only. Consistent bedtimes and wake-up times are recommended in the new guidelines; however, weekday-weekend variations have been widely reported for preschoolers who are attending kindergartens worldwide. Social obligations such as school schedule may interfere with individual circadian preferences for bedtimes, known as chronotype. The discrepancy between biological clock and social obligation is described as “social jetlag”. Beyond sleep duration, social jetlag is associated with increased risk of obesity, metabolic disorders, and mental health problems. Research has indicated that feelings of tiredness induced by short sleep duration lead to lower physical activity level. The influence of social jetlag on daytime activity behaviors, however, has not been examined despite its significant impact on health for young children.

The proposed study will follow up a group of 3- to 5-year-olds (approximately n=300) who were participants in an on-going cross-sectional study. It aims to investigate: (1) the temporal associations of social jetlag with physical activity and sedentary time the following day; (2) the longitudinal associations of social jetlag with activity behaviors over a two-year period. Data collected in the ongoing project that is relevant to purposes of the proposed study, including activPAL-assessed physical activity, sedentary time, bed hours, parent-reported screen time and socio-demographic factors, nap time in kindergartens will be used as baseline measures. Social jetlag is computed as the difference in the midpoint of sleep between school days and weekend days. Follow-up measurements will be taken at the similar time of year in 2020 (two years later after the baseline), comprising children’s chronotype, behavioral problems, TV in the child’s bedroom, and outdoor time reported primarily by parents. The findings will improve the understanding of the movement behaviors synergistically, which will better inform subsequent intervention strategies.