Exposure to sufficient light during the daytime is fundamental for the regulation of the sleep/wake cycle, with the blue part of the spectrum most influential. This thesis explores exposure to environmental blue light and level of illuminance in two populations that experience circadian disruption i.e. older people and young people with autism spectrum disorder (ASD). The aim was to examine associations between blue light exposure, illuminance level and sleep/wake and physical activity patterns. Firstly, an exploratory study was conducted in adolescents with ASD living in a residential school setting aged 13-17 years (n=8). Secondly, a cross-sectional study carried out in two seasons (summer and winter) with a comparative study between seasons of varying light exposure and sleep/wake and physical activity outcomes was conducted in older people aged 72-99 years (n=20). In both studies quantitative measures were used to examine personal light exposure and sleep/wake patterns by use of novel equipment known as an actiwatch. This research demonstrated that objective measures of sleep/wake and light monitoring could be successfully administered in two populations with complex sensory issues. Preliminary findings from the exploratory study in adolescents with ASD indicated that exposure to blue light prior to bedtime was associated with a delay in sleep onset. The methodology developed for participant recruitment and engagement in a study using body sensors proved to be successful. Results for the study in older people suggested that between seasons daytime physical activity, blue light exposure and illuminance levels were significantly higher in summer. Correlated component regression (CCR) was used to investigate predictors of sleep parameters, suggesting morning blue light exposure (a predictor of total night-time sleep), daytime activity level (a predictor of sleep efficiency) and visual function (a predictor of minutes awake during the night) may contribute to sleep quality. The findings from these studies suggested that light exposure and health outcomes, such as physical activity and visual function could be responsible for sleep quality. This has important implications for design and health interventions promoting health and wellbeing, i.e. morning light exposure and time outdoors are important for circadian entrainment and building design and routine should reflect a diurnal light pattern light.