Nocturnin (NOCT) is a NADP(H) phosphatase with highly rhythmic expression peaking in the early dark phase (ZT12) in most tissues in mice. Noct-/- (Noct-KO) mice exhibit a range of tissue-specific phenotypes, but notably are resistant to diet-induced obesity and hepatic steatosis on a high-fat diet. Despite this protection from obesity and fat accumulation in the liver, these mice do not eat less and are not more active, but instead have alterations in energy expenditure that are not well understood. Given the widespread role of NADP(H) as an essential cofactor in numerous metabolic reactions, it is likely that NOCT has distinct tissue-specific effects, making it difficult to understand NOCT’s role in metabolism using a global KO model. Therefore, this proposal will use adipose-, liver-, muscle-, and intestine-specific conditional Noct-KO (cKO) mice, generated via the Cre- loxP system, to study NOCT’s role in metabolism. Aim 1 will determine if loss of NOCT in one tissue is responsible for the body weight phenotype. In this aim, both male and female cKO and control mice will be subjected to a high-fat diet for twenty weeks. Body weight and food intake will be recorded on a weekly basis, energy expenditure will be measured using metabolic cages and body composition measurements will be taken to determine differences in fat and lean mass. At the end of the study, plasma, adipose, liver, muscle, and intestinal tissues will be collected from each cKO line and their respective controls at two time points (ZT2 and ZT14) further analyses including measurement of plasma lipoproteins and plasma adipokine levels. Histology will also be done to define any gross morphological differences caused by loss of NOCT. Aim 2 will examine the gene expression changes in global Noct-KO and cKO mice. We will determine how the transcriptome of each of the four tissues change over the circadian day in the global Noct-KO when fed HFD for 20 weeks and will compare this to the transcriptome of each of the four individual tissues in the cKOs to assess the relative contribution of local NOCT activity on gene expression. In Aim 3 we will examine the molecular mechanisms that result in the protection of hepatic steatosis in the global Noct-KO and will determine whether this liver-specific phenotype is the result of loss of NOCT in the liver or requires contributions from other tissues. The experiments outlined in this proposal will define how tissue-specific loss of NOCT, and thus how tissue-specific increases in NADP(H) levels, impact metabolism. More specifically, this proposal will aid in understanding how loss of NOCT protects against diet-induced obesity and hepatic steatosis, which, in turn, can aid in the development of treatments for obesity and obesity-related diseases.