Project Summary/Abstract American adults have a high prevalence of overweight, obesity and prediabetes. Small weight loss delays the progression to type 2 diabetes and decrease cardiovascular risk, yet adherence to long-term calorie restriction is difficult to sustain. There is an urgency to find effective, easy-to-implement and sustain, and affordable life style interventions. Restricting the food intake interval, or time restricted eating (TRE) has been shown in small-scale pilot studies to result in weight loss and improve metabolism, while being less challenging than calorie count. We propose to rigorously assess the efficacy of TRE, administered via a smartphone application, on weight loss and decreased cardiovascular risk. To achieve this goal, metabolically unhealthy mid-life adults with overweight or obesity who habitually eat for more than 14h/day, will be randomized to a restricted eating window to 10h/d (TRE) or to their habitual eating window (≥ 14h, HABIT), and followed up to 12 months. Ambulatory measures of food intake, sleep, physical activity and glucose, and in-patient 24-h well-controlled studies will be done to determine the effect of TRE versus habitual eating duration (HABIT), as well as the mediators of these effects. Hypotheses: 1) TRE vs. HABIT will result in decreased fat mass, measured by quantitative magnetic resonance, and effect mediated via decreased daily total energy intake, measured by double labeled water; 2) TRE vs. HABIT will result in lower insulin resistance, lower glycemia and shift in fuel utilization preferentially to lipid mobilization; 2) Adherence to TRE (measured by usage of the app and reduction of the eating window), and self-efficacy will associate with the short-term effect (3 months) and the long-term sustainability (12 months) of TRE on fat mass loss. Results from this study will provide important insights into understanding the physiological and molecular interactions between restricting daily eating interval and metabolic function, and could provide evidence for using TRE interventions to improve metabolic health and decrease cardiovascular risk in the large number of mid-life and older Americans in great need of life style intervention.