| Summary: | Background Sleep and physical activity are essential functions of human health. According to International recommendations, adolescents should sleep 8-10 hours each night and engage in moderate to vigorous physical activity for at least 60 minutes per day for overall wellbeing. Previous studies have shown that during adolescence, sleep patterns change and most sleep studies up to date are based on subjective data for sleep duration and quality. Technological advancement in measurement of sleep and physical activity with objective measures adds value to the literature. Aim The main aim of the dissertation was to measure free-living sleep and physical activity in Icelandic youth at baseline (15 years) and two years later (17 years), and explore associations between sleep (duration, quality and timing), physical activity, and body composition. In addition, to compare sensitivity and agreement between subjective and objective measurements of sleep among and between ages of 15 and 17 years old. Methods The study sample came from six elementary schools in Reykjavík. Data was collected during spring 2015 among 315 participants at age 15. In 2017, follow-up data was gathered from 168 repeating participants at age 17. Data from one week of free-living sleep and physical activity was collected with accelerometers. Participants filled out a sleep diary to support accelerometer data and filled out a comprehensive questionnaire on sports participation, physical activity, mental health and parental education, etc. All measurements took place at the research lab (2017) or at individual schools (2015). Body composition was assessed at The Icelandic Heart Association with dual energy X-ray absorptiometry (DXA). Results The majority of adolescents did not get the recommended 8-10 hours of sleep per night according to accelerometer data. This especially applied during the school week (7 hours/night in bed, 6.2 hours/night actual sleeping, about 11% met the recommendation). In comparison, subjective self-reports estimated bedtimes ...
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