Heart rate variability (HRV) appears to be a transdiagnostic biomarker for health and disease. Although initial studies using HRV biofeedback (HRVB) to regulate HRV as a potential adjunctive treatment to gold-standard interventions seem promising, more research is needed to determine which aspects of HRVB training provide the most clinical benefits to those suffering from mental health symptoms. In the current study, we sought to investigate whether time spent in resonance, between-person

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... ences in resonance frequency, and/or within-person resonance frequency trajectory across repeated HRVB sessions were related to changes in depression and/or anxiety symptoms during a 12-week digital mental health intervention that contains HRVB as part of the treatment protocol. We used a retrospective cohort study to examine these associations among 387 participants in the Meru Health Program. For depression, we found that average resonance time per HRVB session, but not total time in resonance, was significantly associated with decreased depression as measured by the Patient Health Questionnaire 9-item scale (PHQ-9) across treatment (b=-0.38, 95% CI [-0.76,-0.01], t(377)=-1.99, p=.047). For anxiety symptoms as measured by the Generalized Anxiety Disorder 7-item scale (GAD-7), we found neither association significant. Within-person effects were significant for both depression and anxiety, with steeper slopes of time spent in resonance significantly related to reductions in PHQ-9 and GAD-7 symptoms, respectively. Between-person effects were not significant for either depression or anxiety. Our results demonstrate that improvements in resonance efficiency over time in treatment, independent of how each participant starts, are related to reductions in depression and anxiety symptoms.