I've been doing my own personal research on HRV and getting to the bottom of it.

The literature is bewildering because of course there are many ways to measure it. If you measure it over the course of the day it is influenced by things like the activities you do. Of course your HRV is going to be higher if you alternate intense activity that raises your heart rate with rest and since activity is so important in it I don't think it is fair to look at a whole day trace.

I think the most important phenomenon is

https://en.wikipedia.org/wiki/Mayer_waves

which are associated with the metric RMSSD as described here:

https://en.wikipedia.org/wiki/Heart_rate_variability

This is called "SD1" in my app

https://gen5.info/demo/biofeedback/

You can use that app to increase the amplitude of your Mayer wave, what you do is breathe in when you see the wave going down and breathe in when you see the wave going up. It is a little tricky if your Mayer waves are initially weak and you might feel light-headed and think "I can't breathe" but once it settles in it is a very strong effect.

I have read a number of patents for HRV biofeedback and they all involve much more complex things that you might think would work if you hadn't tried it but that I don't believe would work having tried it.

Funny I have been taking Nebivolol, another beta blocker, and found that it drastically lowers HRV-inferred stress as measured by my Garmin watch -- I can't really say how it affects my app because I wrote it after I started on the drug.

Cardiorespiratory fitness is associated with lower anger and anxiety