Abstract Obesity is an ongoing epidemic and a serious public health problem. Preventative and therapeutic interventions have mostly focused on diet and physical activity, aimed to understand mechanism and improve efficacy. Notably, there has been a long-term debate over whether macronutrient content in the diet, specifically a low fat vs. low carbohydrate diet can help in management of body mass. Recent insights into the involvement of the circadian system in energy balance control offer a new perspective to understand the relationship between macronutrient intake and weight control. We and others have shown that disruption of the circadian system leads to adverse metabolic effects, including changes in energy balance control mechanisms that can increase the risk for obesity. Indeed, circadian disruption is a recognized critical risk factor for metabolic disorders, as also recognized by the Nobel Prize Committee who awarded the 2017 Nobel Prize in Physiology or Medicine to three circadian pioneers. Given the intimate reciprocal interactions between metabolism and the circadian clock, it has been proposed that dietary macronutrient content may impact whole-body physiology in part through changes in the circadian organization. Consistently, breakthrough animal experimental work has shown that high fat diet leads to disrupted circadian rhythms, including blunted rhythms and lengthened circadian period (cycle length). These findings are consistent with our Preliminary Data in humans, in which we found that higher percentage of 24-h fat intake is associated with (i) a blunted endogenous circadian rhythm in hunger; (ii) a higher caloric intake in the biological evening; (iii) lengthened endogenous circadian period; and (iv) a delayed endogenous circadian phase. Taken together, these results provided strong premise for high fat diet-induced circadian disruption. However, despite the growing realization of the importance of circadian organization and the influence of fat intake on the circadian system, there is very limited experimental data in humans that has tested the effect of high fat diet on the circadian clock or circadian control of metabolism. Therefore, in this proposal, we will address these knowledge gaps by testing the following hypotheses: high fat diet, as compared to low fat diet results in (Aim 1) blunting and delaying of endogenous circadian rhythms of the central clock and measures related to the peripheral clocks; (Aim 2) blunting and delaying in daily and endogenous circadian rhythms in energy intake- related measures; (Exploratory Aim 3) blunting and delaying in daily and endogenous circadian rhythms in energy expenditure-related measures. These aims will be tested in a within-subject, randomized, crossover design in healthy individuals undergoing two protocols. Each of both protocols has a short-term field-based dietary intervention (2 weeks to avoid substantial weight change) followed by a 6-day highly controlled in- laboratory vi...