We have done work in this field for 30 years now. Heart rate variability (HRV) actually is a term that was not chosen well, as it implies an instability of regulation and was treated like this for a long time eg. by cardiologists. In a popular cardiology textbook, HRV is called ' a not-treatworthy heart rhythm disturbance' which is thought to disappear with age.

Actually, a better term would be 'heart rhythm flexibility' (HRF), as the phenomenon is an expression of the regulatory ability of the autonomic nervous system to cope with different ambient situations and especially to interact with other body rhythms like respiration, peripheral blood perfusion and blood pressure rhythmicity. So let me use this term HRF.

Low HRF is known to predict bad outcome in several cardiovascular diseases and even total mortality. HRF is reducing with age but unusually high values can be found in centenarians, implying that HRF i good for life expectancy. Healthy interventions like moderate running, omega 3 fatty acids, stop smoking, meditation increase HRF in the aftermath. On the other had, many bad things for health life like fine particulate matter, heavy metals, stress, reduce HRF in a matter of minutes.

See some considerations on HRF in our papers:

Article Heart rate variability as a prognostic tool in cardiology. A...

Article Adaption of cardio-respiratory balance during day-rest compa...

Article Cardioautonomic Control and Wellbeing due to Oscillating Col...

Article Phase‐ and frequency coordination of cardiac and respiratory function

Article The symphony of life - Importance, interaction, and visualiz...

Article Why Life Oscillates – from a Topographical Towards a Functio...

Article Oscillations of heart rate and respiration synchronize durin...

Article Changes of respiratory sinus arrhythmia during the menstrual...

Article Major depression and cardiac autonomic control

and advanced methods to disentangle HRF components in these papers

Article Disentangling Respiratory Sinus Arrhythmia in Heart Rate Var...

Article In vivo cardiac phase response curve elucidates human respir...

I wish you good luck for your thesis!

Max

There are many conceptualizations of well-being, so it depends on what you want to consider exactly. The Positive Psychology model might be of interest (see Peterson, Park, & Seligman, 2005; doi: 10.1007/s10902-004-1278-z)

As regards HRV, Chiara Cosentino 's papers may be of help (e.g., Article Heart Rate Variability: What if Mind-Body Integration Lies i...

Good luck!

Thank you so much for your feedback, I appreciate it a lot!!