Journal of Medical Internet Research - International Scientific Journal for Medical Research, Information and Communication on the Internet #Preprint #PeerReviewMe: Warning: This is a unreviewed preprint. Readers are warned that the document has not been peer-reviewed by expert/patient reviewers or an academic editor, may contain misleading claims, and is likely to undergo changes before final publication, if accepted, or may have been rejected/withdrawn. Readers with interest and expertise are encouraged to sign up as peer-reviewer, if the paper is within an open peer-review period. Please cite this preprint only for review purposes or for grant applications and CVs (if you are the author). Background: College students experience high levels of stress. Mindfulness meditation delivered via a mobile application (app) may be an appealing, efficacious way to reduce stress in college students. Objective: To test the initial efficacy and sustained effects of an 8-week mindfulness meditation mobile-app (i.e., Calm) compared to a wait-list control on stress, mindfulness, and self-compassion in college students with elevated stress. We also explored the intervention’s effect on health behaviors (i.e., sleep disturbance, alcohol consumption (i.e., binge drinking), physical activity, healthy eating (i.e.,fruit and vegetable consumption) and examined the feasibility and acceptability. Methods: This study was a randomized, wait-list control trial with assessments at baseline, post-intervention (8-weeks) and follow-up (12-weeks). Participants were eligible if they were current full-time undergraduate students and 1) at least 18 years of age, 2) scored ≥14 on the Perceived Stress Scale (PSS), 3) owned a smartphone, 4) willing to download the Calm app, 5) willing to be randomized and 7) able to read and understand English. Participants were asked to meditate using Calm at least 10 minutes per day. A p-value of ≤ 0.05 was considered statistically significant. Results: Eighty-eight participants were included in the analysis. The mean age (SD), for the intervention group was 20.41 (2.31) years and 21.85 (6.3) years for the control group. There were significant differences in all outcomes (stress, mindfulness, and self-compassion) between intervention and control groups after adjustment for covariates (all P<0.04) at post-intervention. These effects persisted at follow-up (all P<0.03), except for the non-reacting subscale of mindfuless (p = 0.079). There was a significant interaction between group and time factor in perceived stress (p = 0.002), mindfulness (p<0.001), and self-compassion (p<0.001). Bonferroni post-hoc tests showed significant within-group mean differences for perceived stress in the intervention group (p0.19). Similar results were found for mindfulness and self-compassion. Effect sizes ranged from moderate (0.59) to large (1.24) across all outcomes. A significant group x time interaction in models of sleep disturbance was found but no significant effects were found on other health behaviors. The majority of students in the intervention group reported Calm was helpful to reduce stress and stated they would use Calm in the future. The majority was satisfied with using Calm and likely to recommend Calm to other college students. Participation in meditation for the intervention group averaged 38 minutes/week during the intervention and 20 minutes/week at follow-up. Conclusions: Calm is an effective modality to deliver mindfulness meditation to reduce stress and improve mindfulness and self-compassion in stressed college students. The findings presented here provide important information that can be applied to the design of future studies or mental health resources in university programs. Clinical Trial: clinicaltrials.gov, NCT03891810; https://clinicaltrials.gov/ct2/show/NCT03891810?term=calm+college&rank=1 (archived by WebCite at http://www.webcitation.org/77MgrQjDj