Mental health and physical activity in adolescence

Sammanfattning: The aim of this thesis is to examine the association between physical activity and mental health among Norwegian adolescents.The thesis includes four studies among adolescents in grade 10, aged 15-16 years. Studies I, II and III are based on the Norwegian Youth Health Surveys and study IV is based on data retrieved from Ungdata. In 2000-2003 (Youth Health Survey), the adolescents completed a paper and pencil self-administered questionnaire at school during lesson time. In both 2009 (Youth Health Survey) and 2017 (Ungdata), the adolescents completed an anonymous web-based questionnaire.The Rasch analysis from study I showed that overall, the HSCL-10 showed good reliability and the items worked well. One item “Sleeping difficulties”, in the HSCL-10 instrument clearly misfit and some items worked differently for boys and girls. Study II showed that the association between physical activity, screen-based sedentary behaviour and psychological distress was weak. In study III, a significant association between physical activity and incident use of hypnotics was shown at short-term follow up. No significant associations were found for later use of antidepressants. Study IV showed that participating in a sports club was significantly associated with lower odds of depressive symptoms.Mental health and physical activity are both complex phenomena. When investigating the association between adolescent`s physical activity and mental health, it is important to look at physical activities in different contexts, not only volume and frequency.