The study includes 2 weeks of monitoring of the participants' patterns of sleep and wakefulness at home. At the beginning of this period, all participants will undergo measurements of their body composition (lean tissue, bone and fat) and their metabolic rate at rest and after a standard meal. At that time, all participants will also drink a glass of water containing harmless non-radioactive dense forms of oxygen and hydrogen, small amounts of which are found in natural water. Several urine samples will be collected before and after the participants drink the water at the beginning of the study and again 14 days later in order to measure the amount of energy that was used by them during this time. Participants will also wear small wristwatch-like activity monitors on their wrist and around their waist as they follow their usual everyday activities and sleep-wake schedules at home. The recordings from these monitors combined with daily sleep logs will be used to determine when the participants slept and when they were awake. On the last day of the study, the participants will undergo MRI measurements of the distribution of fat in their abdomen and complete a graded exercise test on a treadmill or stationary bicycle in order to determine what is the most strenuous level of exercise that they can perform. A medical doctor will supervise all study procedures that are done in the research laboratory.