As the share of older adults is growing at a high pace, so is the number of people living with cognitive decline. To reduce the prevalence of cognitive decline, research on healthy cognitive ageing is needed. Among older adults, engaging in physical activity is positively associated with maintenance of optimal cognitive functioning. Nevertheless, the underlying pathways via which physical activity influences older adults’ cognitive performance are poorly understood. Two often proposed mediators of the protective effect of PA on cognitive performance are sleep and mental wellbeing. However, empirical evidence for the mediating effects is scarce and a number of methodological issues, including the focus on associational rather than causal analysis, the lack of attention for within-person effects at day-level and the lack of research performed in older adults’ habitual environment, has received little attention. The aim of the current proposal is to investigate, using sophisticated statistical methods and by collecting high-quality data on a daily level, whether physical activity causes improved cognitive performance via improved sleep and elevated mental wellbeing among older adults. In the first phase of the project the causal structure between physical activity, cognitive performance, sleep and mental wellbeing will be investigated. In the second phase the direct and indirect effects between physical activity and cognitive performance will be quantified.