This study will test whether people can change the number of hours they sleep at night and if such an increase in sleep affects their body weight. It will examine whether the number of hours of sleep relate to body weight, the amount of body fat and the level of hormones that help to control appetite.

People between 18 and 50 years of age who sleep less than 6 hours at night and who are obese may be eligible for this study. Each candidate is screened with a medical history and physical examination, psychological interview, blood and urine tests, electrocardiogram and questionnaire about sleep, mood, physical activity and level of sleepiness and appetite. For two weeks, participants will keep sleep and exercise diaries and wear a wrist device (activity watch) that measures how much they move around throughout the day and night. They keep a food diary for three days and then are admitted to the hospital for a sleep study. For this test, electrodes are attached to the subject's chest, chin, scalp and skin near the eyes. A band is placed around the chest, a small device is placed near a nostril, and a clip is placed on the index finger. These devices are used to record information about breathing, sleep stages, electrical activity in the brain and heart, and blood oxygen levels. A video camera records the subject's movements.

Participants are randomly assigned to one of two groups. Over the 12-month study, group A subjects increase their amount of night-time sleep up to a limit of 7.5 hours a night; group B subjects continue to sleep the same amount they have been sleeping in the past 6 months. Plans are devised for group A to increase their sleep time and for group B to continue getting the same amount of sleep they have in the past. All subjects undergo the following tests and procedures:

• Baseline visit - Month 0 (overnight hospital stay): At the start of the study, participants have blood and urine tests, their sleep, exercise and food diaries are reviewed, and their body measurements are taken. They have various tests that measure their body fat and lean muscle tissue, the rate at which their body burns calories, and their level of physical conditioning. They answer questionnaires about their appetite, sleepiness, mood, anxiety, pain, and health status and are tested for diabetes and daytime sleepiness. Between visits, subjects fill out exercise and sleep diaries, wear the activity watch, record their weight and blood pressure, and wear a pedometer to record a daily count of the steps they take.
• Interim visits - Months 1, 2, 4, 6, 8 and 10: Participants have blood tests and their sleep and exercise diaries, food intake from the previous day, blood pressure and weight measurements, pedometer step records and activity records from the activity watches are reviewed. They are asked about their sleep, mood and anxiety, and they rate their appetite and sleepiness.
• End-point visit - Month 12 (2-day hospital stay): Participants repeat the sleep study and the tests done at the baseline visit. They are given information about a healthy diet, exercise and sleep. They are asked to exercise regularly, eat a healthy balanced diet, continue to wear the pedometer, complete the sleep and exercise diaries for one week of each month, and take monthly weight and blood pressure measurements. Group A subjects are asked to continue sleeping as much as possible up to 7.5 hours a night; group B subjects are given a plan to try to increase their sleep up to 7.5 hours a night.
• Follow-up visit - Month 18 (2-day hospital stay): Participants repeat several, but not all, of the tests and procedures they had at the 0- and 12-month visits, have another sleep study, and review the sleep, exercise, and food diaries for the last 6 months.