HRV is a meaningful, non-invasive measure of autonomic nervous system health and is associated with survival, stress management and mental health. Slow breathing is the best-supported method for increasing HRV; HRV biofeedback has supplementary value for anxiety and depression. Causal relationships have not always been established.
Heart rate variability (HRV) is the variation in the time between successive heartbeats. That variation is not an error or noise, but a sign of a flexible and well-functioning autonomic nervous system -- the part of your nervous system that regulates heart rate, breathing and digestion without you having to think about it. HRV is regarded as a non-invasive measure of the activity of the vagus nerve, the main inhibitory nerve in your body. A higher HRV is linked in multiple studies to better adaptability to stress and to better overall health, but it is a signal, not a direct cause of health (PMID 39625562, 34710127).
One retrospective cohort study (n=271) in hospitalised COVID-19 patients illustrates how clinically relevant HRV can be: in patients aged 70 and older, a higher HRV at admission predicted a greater chance of survival (hazard ratio for death: 0.53) and a lower chance of referral to the intensive care unit in the first week (hazard ratio 0.51). This association was not found in younger patients. This is observational research, so no causal relationship can be derived from it. Nevertheless, it shows that HRV can provide clinically relevant information in vulnerable older adults beyond the usual risk factors (PMID 34710127).
Good news: you can actively influence your HRV. A large systematic review and meta-analysis of 223 studies convincingly demonstrates that voluntarily slow breathing increases vagally mediated HRV, both during the breathing session itself, immediately afterwards, and following a programme of multiple sessions. It is an inexpensive technique with few expected side effects. This is the strongest evidence for direct modifiability of HRV among the available claims (PMID 35623448).
HRV biofeedback, in which you learn to consciously control your heart rhythm using a device, was examined in a meta-analysis of 58 randomised controlled studies. The effect on anxiety, depression, anger and athletic performance was small to moderately positive, comparable to other existing treatments. For PTSD, sleep and quality of life the effects were smaller. The researchers view HRV biofeedback primarily as a complementary rather than a replacement treatment (PMID 32385728). A smaller randomised study (n=76) compared five weeks of HRV biofeedback, mindfulness meditation and physical activity as self-help interventions. All three reduced stress, anxiety and depressive symptoms and improved sleep quality, with no significant difference between them (PMID 26111942).
Finally, researchers have investigated whether music can influence HRV. Different music genres can stimulate or inhibit the autonomic nervous system, but the effects are variable and highly dependent on the type of music. The researchers conclude that the methodological quality of the available studies is too low to recommend music intervention as a serious cardiovascular therapy (PMID 34237410). In short: HRV certainly tells you something about your health, but it is one signal among many, and you can consciously work on it through breathing exercises or biofeedback.
The claims are based on one large meta-analysis on slow breathing (223 studies, PMID 35623448), one meta-analysis on HRV biofeedback (58 RCTs, PMID 32385728), one RCT (n=76, PMID 26111942), one retrospective cohort study on COVID-19 (n=271, PMID 34710127), one review on music (PMID 34237410), and one review article on HRV as a biomarker (PMID 39625562). Most associations are correlational; for slow breathing there is stronger causal evidence.