Chronic sleep deprivation may constitute an important and potentially correctable behavioral factor in the alarming increase in obesity. There are no definitive experimental studies in humans showing whether sleep deprivation indeed contributes to increased energy intake and/or reduced energy expenditure. The investigators propose a series of novel studies to investigate abnormalities in energy homeostasis imparted by sleep deprivation. The investigators will measure food intake, energy expenditure (basal metabolic rate, thermal effect of food, and non-exercise activity thermogenesis), and neurohormone levels in 24 healthy subjects with normal BMI (20-25 kg/m2). Twelve subjects (6 men and 6 women) will be randomized to sleep deprivation. Measurements will be compared to those obtained in 12 subjects who are randomized to a control group, and are not sleep deprived. The investigators will test the following hypotheses: 1. That sleep deprivation results in positive energy balance (increased caloric intake and decreased energy expenditure, as reflected by decreased non-exercise activity thermogenesis). 2. That dysregulation of appetite and energy expenditure is associated with changes in molecules controlling appetite and metabolism. 3. That changes associated with 8 days of modest sleep deprivation resolve, at least in part, over a 4 day recovery period.