The aim of this study was to determine the effects of a personalized smartphone application on subjective sleep quality, wellbeing and stress in participants with subclinical sleep problems. Healthy participants with subclinical threshold sleep issues that did not meet diagnostic criteria for insomnia were randomly assigned to use the app or be on a waiting list for app usage (control group). Subjective data on sleep quality (sleep quality component of Sleep Questionnaire B [primary endpoint] and the Pittsburgh Sleep Quality Index), feeling refreshed after the sleep component of Sleep Questionnaire B, Short Form-12 physical and mental components, and stress (Perceived Stress Scale) were compared between the app and control groups at baseline and after 6 and 12 weeks. Depending on their usage, app users were defined as "frequent", "occasional" or "rare/never" users. Two-hundred and six and 205 app users and 286 and 285 control participants completed the 6- and 12-week assessments, respectively. Sleep Questionnaire B scores improved in frequent (effect size 0.595
F = 13.7) and occasional users (0.653
F = 9.4) after 6 weeks
effects persisted at 12 weeks, and were similar for the Pittsburgh Sleep Quality Index. Feeling refreshed after the sleep component of Sleep Questionnaire B improved over time in all groups, with no significant difference between app users and controls. There were no changes in Short Form-12 physical component, Short Form-12 mental component, and Perceived Stress Scale scores. A personalized app positively affected subjective sleep quality in a population of users with subclinical threshold sleep issues, and could serve as a low-burden digital intervention to potentially prevent the exacerbation of sleep pathologies.