Mental well-being during middle childhood and early adolescence : associations with perceived social support and 24-hour movement behaviours

Mental well-being challenges frequently emerge during childhood and adolescence and can disrupt developmental trajectories, with lasting consequences for health, education, and social functioning. Alongside the prevention of negative mental well-being indicators such as anxiety and sadness symptoms, promoting positive mental well-being, including optimism, life satisfaction, and self-esteem, is increasingly recognized as a critical component of youth mental health. Social support from family and peers and the 24-hour movement behaviours, including physical activity, sedentary behaviour, and sleep, are each associated with youth mental well-being; however, limited research has examined how these factors jointly shape mental well-being across middle childhood and early adolescence. Using longitudinal and cross-sectional data, this dissertation addresses three objectives: (1) to characterize developmental changes in negative and positive mental well-being indicators, including anxiety and sadness symptoms, optimism, life satisfaction, and self-esteem, and to examine moderation by sex and social support across middle childhood and early adolescence; (2) to identify distinct movement behaviour patterns in middle childhood and assess their longitudinal associations with mental well-being, including whether these associations are moderated by sex and social support; and (3) to examine how the relative distribution of time across sleep, sedentary behaviour, light physical activity, and moderate-to-vigorous physical activity is associated with mental well-being before the transition to secondary school, using a compositional data analysis approach. Mental well-being emerged as developmentally dependent, with positive and negative indicators following distinct trajectories during middle childhood and early adolescence, and as context-dependent, shaped by daily movement behaviours and social relationships. Movement behaviours were linked to positive and negative mental well-being indicators not only as individual activities but also as part of integrated daily patterns. Social support further influenced these relationships, at times strengthening favourable associations and, in other cases, buffering against the adverse effects of less optimal movement behaviour patterns. Overall, the findings underscore the importance of social relationships and the interdependent nature of 24-hour movement behaviours for youth mental well-being, informing developmentally appropriate interventions that support mental well-being during childhood and early adolescence.