The effect of physical activity and sedentary behaviour on mental health amongst young adults.

Through a systematic review, cross-sectional and longitudinal studies, the current thesis found that physical activity was associated with reduced mental health symptoms; however, this effect was not maintained after multivariate control for young adult psychosocial characteristics. Further, engagement in long periods of sedentary behaviour (sitting time) was associated with higher psychological distress in the cross-sectional study, but not related in the longitudinal study. A significant contribution of the thesis was the finding of a threshold at which sitting time may be related to mental health.