Due to insufficient treatment options for sleep disorders, effective solutions are urgently needed. We evaluate the effects of a CBT-I-based app combining sleep training with subjective and objective sleep monitoring on i) sleep and ii) subjective-objective sleep discrepancies (SOSD) as known for insomnia. Fifty-seven volunteers (20-76 years; MAge=45.67±16.38; 39 female) suffering from sleep problems were randomly assigned to an experimental group (EG, n=28) or a waitlist control group (CG, n=29). During the 6-week app phase, the EG used the CBT-I-based program and a heart-rate sensor for daily sleep monitoring and -feedback, while the CG used the sleep monitoring only. Sleep was measured i) subjectively via questionnaires (Insomnia Severity Index, ISI; Pittsburgh Sleep Quality Index, PSQI), ii) objectively via ambulatory polysomnography (PSG), and iii) continuously via heart-rate sensor and sleep diaries during the app phase. Data revealed interactions for ISI (p=.003) and PSQI (p=.050), indicating training-specific improvements in EG, yet not in CG. While PSG-derived outcomes generally appear to be less training-specific, a statistical trend towards a reduction in WASO was found in EG (p=.061). Regarding changes in SOSD, results indicate improvements during app phase (EG) for SE, SOL and WASO (p’s ≤.022); for TST both groups show a SOSD reduction. The findings indicate beneficial effects of a novel smartphone app on sleep and SOSD. More scientific evaluation of such digital programs is needed and should ultimately help to reduce the gap in non-pharmacological treatment care for insomnia.