The main aim of this study is to examine the effects of restricted sleep on energy and substrate metabolism as well as relevant hormonal systems that might be involved in the underlying mechanisms.

The investigators hypothesize that short-term sleep curtailment in adolescents is accompanied by 1) increased hunger and desire to eat; 2) increased spontaneous energy intake; 3) reduced spontaneous physical activity; and 4) increased fluctuation in hormones in a way that could favour a positive energy balance and potentially weight gain on the long run.

The background for this project is that despite considerable knowledge of the mechanisms regulating body weight, the prevalence of obesity has not declined. The individual and environmental factors that play a part in energy balance are still not completely understood, and current methods for loosing and maintaining body weight are generally unsuccessful. Knowledge about what brings about changes in metabolism and lifestyle is lacking.

Reduced sleep duration has been observed in parallel with increases in body weight. Lack of sleep has become a hallmark of modern man, and both children and adults sleep less today than just a few decades ago. Research suggests that both too much and too little sleep can lead to obesity, but correlations between sleep deprivation and other lifestyle-related ailments have also been observed. The reasons for these correlations are far from clear, especially in children and adolescents.

The main purpose of SLOPUS is to investigate the effects of reduced sleep on energy and substrate metabolism, and the relevant hormonal systems involved in the underlying mechanisms. Our hypothesis is that acute sleep deprivation in teenagers leads to increased hunger and desire to eat, increased spontaneous energy intake, decreased spontaneous physical activity, and increased fluctuation in the hormones that control energy metabolism.

The investigators aim to recruit twenty-four healthy, normal weight boys aged 15-19 years with normal sleep pat-terns. The study will be conducted as a randomized, crossover study with two conditions. Each participant will be subjected to short sleep (4 hours) and normal sleep (9 hours), in random order, for three consecutive nights in a respiration chamber. They will stay in the chamber for the 24 hours before the last night. In the respiration chamber energy turnover is measured - both how much energy and also the type of substrate combusted. The chamber is equipped with a bed, toilet, television, computer, and communication. Food and drink can be supplied via an airlock. Measurements in the chamber include volitional activities and sleep duration/quality.

Energy turnover will be measured using a ventilated hood system on the first and last day. Here the participants lie down on a bed with their heads under a transparent canopy for 25-minute intervals during the morning. Blood samples will also be taken during these periods. Meal tests will be served, where we measure the amount of food ingested, and subjects will be given questionnaires about hunger and satiety to fill out. Physical activity and heart rate will be measured throughout the trial.

Thus, the present study is likely to result in a number of new and important findings regarding the impact of impaired sleep on energy balance and it might improve the recommendations for optimal sleep in adolescents.

Inclusion Criteria:

• normal weight (5th percentile to less than the 85th percentile)

Exclusion Criteria:

• self-reported sleep problems (Pittsburgh Sleep Quality Index score >5);
• night work;
• variable sleep habits or habitual daytime naps;
• regular physical exercise (>3 hours/week);
• excessive intake of alcohol (>7 drinks/week);
• excessive intake of caffeine (>300 mg/day);
• smoking;
• substance abuse;
• use of prescription medications or over-the-counter drugs affecting sleep or metabolism;
• chronic diseases (e.g. cancer, asthma, back pain, thyroid disease, heart disease, diabetes, etc.)
• history of neurological, medical or sleep disorders;
• trans-meridian travelling the last month;
• eating disorder;
• irregular eating schedule (e.g. skipping breakfast);
• unable to comply with the protocol.