I’ve only found two things that made a difference, and both can be distilled into “Spend more minutes per day above 150 BPM.”

One was buying an elliptical and performing HIIT workouts[1]. After 3-4 weeks, that correlated with a roughly 4 ms increase in daily average HRV. I already was in good cardio shape and rode a spin bike daily[2], but cycling was mostly 120-150 BPM, not 150-175.

The second was adding an additional ~150 calories/day of workouts, mostly intense. In my case, it was a mix of more elliptical sessions, high-BPM cycling, and Beat Saber VR on the hardest difficulty - basically intense aerobics - but I have no reason to think the specific exercises mattered. That correlated with an additional 6 ms increase in HRV.

For a month, keep track of how many minutes per day that you spend above about 80% of your max HR. That's above about 150 BPM for a 40 year old[3]. In that month, try to spend at least 1 minute every day above 160 BPM. See whether it increases your average HRV.

Usual disclaimers apply, particularly that these are N=1, not at all blind, and observational (I was trying to increase my cardio fitness, but did not expect it to manifest in HRV). The correlations are really strong, though. I wish iOS Health tracked minutes per day of intense cardio; I think other people would notice a correlation.

Also, I’ve had other periods where I did a lot more zone 1-2 fitness (roughly 90-130 BPM) and didn’t see any noticeable impact to HRV, even though I felt like I was in at least a bit better cardio fitness. That may be because my existing exercise was already delivering the benefits that low-intensity exercise could deliver, or maybe low-intensity just isn't enough to noticeably move HRV for me.

[1]: “Sprint 8”: https://sprint8.com/ . 30 seconds all-out, 90 seconds recovery, 8 times. If you haven’t done HIIT before, you should be exhausted after the 3rd or so rep and unable to do full-speed sorints after 5 or 6.

[2]: It’s very possible that the “zone 3” cycling cardio also increased my HRV, but from my POV it was in my baseline. I don’t have before and after comparisons.

[3]: Heart rate targets by age https://www.heart.org/en/healthy-living/fitness/fitness-basi...