The purpose of this study is to determine whether there are diurnal differences in postprandial beta-cell function and hepatic insulin sensitivity and the factors that influence these metabolic functions, including insulin signaling, adipose tissue and systemic inflammation, nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide(NAD) biosynthesis, and sirtuin (silent mating type information regulation 2 homolog 1 (SIRT1)) in overweight human subjects.