The study investigates sleep structure and its development from birth to adolescence, assessing how child and maternal sleep, stress, and socioeconomic factors influence child development and health. Pregnant women recruited from four hospitals in the Capital Region of Denmark (2026–2027) will be invited at their mid-pregnancy ultrasound (week 18–22). Participants and their children will be followed from pregnancy through childhood with repeated assessments of somatic health, neuropsychological functioning, sleep (accelerometry, PSG, questionnaires), stress (questionnaires, hair cortisol), and family functioning. MRI scans of the child's brain will be performed at defined developmental stages (infancy, 1.5 years, from age 4, and every two years thereafter). Data will be linked to national health and education registries for comprehensive longitudinal analysis. Findings aim to enhance screening, clinical recommendations, and health strategies for better sleep habits.