 Brown adipose tissue is poorly understood fat that can metabolize glucose in order to generate heat. Since activated brown fat has a high metabolic rate, it is of great interest as a potential target to combat obesity. However, the signaling and control of brown fat metabolism is poorly understood. Because brown fat uses glucose as its energy source, brown fat metabolism can be imaged with PETCT using the positron emitting glucose analog F-18 FDG. We have recently shown in mice a striking circadian variation in brown fat metabolism as evidenced by changes in FDG uptake. In this study we endeavor to generate pilot data on a potential circadian variation in brown fat activation in healthy humans. 