I have an Apple Watch. I don’t pay attention to my HRV on a daily basis, but if I look at it I can often see in retrospect—for example I had a cold at the beginning of the month which clearly shows up as a ~30ms drop in HRV.
I've been tracking HRV since 2018 (I wear an Apple Watch 24/7, aside from charging).
Over 6 years between 2018-2024, I went from being 70 lbs overweight / very out of shape, I then lost all of the weight in 12 months, started going to the gym, became physically fit, and I continue to exercise by cycling 3-4 days a week (sustained heart rate 165 for at least an hour).
Despite going from basically an overweight couch potato to someone very physically fit and strong cardio system from cycling, there has been virtually zero noticeable trend with HRV.
On the flip side, I've noticed resting heart rate correlates extremely closely with cardio fitness and stress. Resting heart rate tends to go up in stressful months or months I don't workout as much. HRV seems to just always be a flat line.
I'm also very skeptical of Vo2 max. For some reason, the watch only records maybe 2 or 3 data points per month at most. Sometimes it doesn't record anything for an entire month even though I'm doing high intensity cardio weekly. Apple rates my Vo2 max as "below average" and similar to HRV, it's a flat line. Seems to not be correlated with my perceived fitness level.
I haven't noticed any correlations with being sick. My impression of all of these metrics (including resting heart rate) is they're maybe useful, but only in the long-term to see trends over time. They aren't very useful for predicting short-term health events, in my experience.
VO2 max is only measured when you do a “brisk walk, hike or run outdoors” according to the Health app. So if you’re doing any other workout type it won’t measure this metric.
Maybe I should pay more attention to it.