Background: College students are at increased risk of poor mental health and well-being as well as poor sleep. There is an ongoing need for accessible interventions for young adults in this important period of transition. Nature-based interventions have been proposed as scalable strategies to reduce stress and improve health, yet few randomized controlled trials have assessed their longitudinal effects on both mental health and physiological functioning.Methods:We conducted a 14-week randomized controlled trial conducted from January 21, 2023, to May 5, 2023 among college students at a single university (N= 379). Participants were randomized to the control group or one of three interventions: physical activity, nature experiences, or group therapy. Each intervention offered weekly group activities and prompted intervention-specific individual activity suggestions. The physical activity group completed weekly exercise routines; the nature experiences group spent quality time in nature each week; and the therapy group engaged in skill-based group therapy. Individual activities were scheduled through the study app. Weekly self-reported health measures included the Depression, Anxiety, and Stress Scale (DASS-21), the Perceived Stress Scale (PSS), and the Warwick-Edinburgh Mental Well-being Scale (WEMWBS). Physiological sleep measures were continuously recorded via Oura Rings. We used an intention-to-treat analysis, including participants that attended at least one group activity, and mixed-effects models to assess intervention effects over time. To explore potential mechanisms, mediation analyses were conducted to assess …