 RESEARCH DESIGN: The study is a single-center, parallel randomized controlled trial with two study arms intervention and control. The intervention arm will use Lark, a mobile coach app, which provides a just-in-time adaptive intervention JITAI by providing motivational feedback, goal setting, and emotional social support, among other change techniques. The control group will use MyFitnessPal, a calorie-counting app, which does not include JITAI components, but allows users to keep track of their caloric intake and energy expenditure. 