Our study investigates the possible association between the physical living environment in children’s living environment and their circadian functioning during the first 12 years of their lives. Outcome measures are the following aspects of child circadian functioning: 1) circadian cortisol markers and 2) total sleeping problems. Data consists of salivary cortisol samples that were collected to assess the circadian functioning of the hypothalamus-pituitary-adrenal axis and questionnaire data to assess sleep quality. Physical living environment will be operationalized as Open Green Spaces, Sky View Factor, and Tree Cover Density, at different radii around the child’s home (200 m, 500 m, 1.000 m, and 2.000 m). We will perform three separate multilevel model analyses to investigate the associations between: 1) Physical living environment and area under the curve of circadian cortisol at ages 2.5, 6, and 10 years, 2) Physical living environment and the slope of circadian cortisol at ages 2.5, 6, and 10 years, ...