This primary goal of this pilot project is to obtain preliminary data demonstrating the feasibility of measuring lipolysis and fat oxidation during sleep in adult human subjects using microdialysis, stable isotope labeling techniques, and whole-room indirect calorimetry. The results of this study will be used in grant applications to perform definitive studies using these methods. The long term goal of this line of research is to explore the mechanisms responsible for sex based differences in the effectiveness of physical activity as an approach to weight loss. Previous longitudinal data have shown that in normal weight and obese individuals, for any given increase in physical activity, there is a significantly greater loss of fat mass in men relative to women [Westerterp, 1997]. These data need to be reconciled, however, with the wealth of data demonstrating that during exercise, women release and oxidize fat more readily than men [Horton, 2009; Horton 1998, Moro 2007]. This phenomenon seems counterintuitive, given that men have less body fat than women and are able to lose more body fat than women when beginning a weight loss program [Bjorntorp, 2007]. Recent data suggests a mechanism to explain this paradox. Specifically, in obese subjects, 24-hour fat oxidation increased following a daytime bout of exercise, and this increase occurred in men only and was driven by a significant increase in fat oxidation not during the day but rather during sleep the night following exercise [Bergouignan, 2014]. Since the predominant source of fuel during sleep is free fatty acids (FFA) released from adipose tissue (i.e., lipolysis), it may be that enhanced sleeping lipolysis is a key factor in how exercise stimulates fat oxidation and thus greater fat loss in males vs. females over time.