How strong is the evidence for HRV?

A 2017 meta-analysis found HRV biofeedback training — deliberately practicing slow, paced breathing to increase heart rate variability — produces a meaningful reduction in self-reported stress and anxiety, a mechanism-consistent finding supported by a separate meta-analysis showing voluntary slow breathing reliably increases HRV. That evidence is currently rated moderate (70/100), built on 9 published studies, including 5 randomized controlled trials and 4 meta-analyses -- a reasonable base, but not yet enough for full certainty.