Project Summary More than 1 in 4 children receive at least one psychiatric diagnosis (PD) before age 17. These youth face significantly elevated chronic disease risk including high prevalence of obesity, which is propelled by low physical activity (PA) levels, poor sleep, and social isolation. Unhealthy PA and sleep behaviors not only increase preventable chronic disease risks, they also worsen the cognitive and behavioral functioning challenges these youth experience. The goal of “GamerFit” is to test the delivery of a theory-based mHealth app that utilizes social support, exergaming, and telehealth coaching to improve PA levels, sleep, and psychiatric symptoms among youth participants (ages 14-17 y) with PD. In order to aid future intervention optimization, we will randomize 60 participants with at least one PD, with 30 using the GamerFit app with weekly telehealth coaching sessions and 30 using a commercial healthy habits app as a comparator group. The GamerFit app will provide access to a menu of exergames, progressive PA and sleep plan, integrated self- monitoring, pre-recorded videos addressing healthy PA and sleep habits, and motivational text messages to help users meet PA and sleep guidelines over 12 weeks. The project leverages our pilot data on exergaming and telehealth coaching to improve PA in youth with PD, and our interdisciplinary team has expertise in developmental psychology, PA and sleep intervention design, technology-based health promotion, and app development. The major research question is: Is a 12-week exergaming and telehealth coaching intervention, delivered via a smartphone app, an acceptable, feasible and effective method to improve PA and sleep behaviors in youth with PD? The specific aims are as follows: Specific Aim 1: To examine feasibility and acceptability of the 12-week GamerFit intervention delivered through a mobile app to youth with PD and their parents. Specific Aim 2: To test the hypothesis that the 12-week GamerFit intervention delivered through a mobile app will improve PA levels and sleep quality and duration compared to a comparator group that uses an unstructured healthy habits tracking app. Exploratory Aims: To test the hypothesis that the 12-week GamerFit intervention delivered through a mobile app will improve mood, behavioral regulation, and perceived social isolation and support compared to the comparator group; and to test the hypothesis that behavioral effects will be sustained at week 16 (i.e. 4-weeks after the end of the intervention). We will also work with clinicians to explore approaches to incorporating the app, specifically PA prescriptions, into psychiatric treatment plans. This project impacts the field by testing the delivery of a theory- and evidence-based exergaming and telehealth coaching intervention targeting multiple health behaviors in youth with PD via a low-cost app.