Study participants will complete two 16-day inpatient stays in the sleep laboratory of the University of Chicago Clinical Resource Center that will be scheduled at least 4 weeks apart. Bedtime duration will be set at 5.5 hours per night during one of these stays and at 8.5 hours per night during the other. No daytime naps will be allowed. Study participants will be served regular daily meals including breakfast, lunch, dinner, and a bedtime snack. On weekdays all participants will engage in simulated "office work" in the sleep laboratory. During the rest of the time participants will maintain their usual indoor and outdoor activities as much as possible within the limits of the University of Chicago campus. At the beginning of each inpatient stay, study participants will 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 and again 14 days later in order to measure the amount of energy that was used by them during this time. At the end of each stay participants will also undergo measurements of their body composition (lean tissue, bone and fat) and their metabolic rate at rest and after a standard meal, and will complete tests to determine how much insulin does their body produce in response to an intravenous glucose infusion and how effective is the action of the sugar-processing hormone, insulin, in their body when it is infused intravenously together with glucose over a period of several hours.