I think Heart Rate Variability data (good overview from Harvard Medical Health Blog) can be a strong indicator of health relative to an individual’s baseline. The problem for me has always been “baseline.”
In the early days of HRV, I measured my beat-to-beat measurements using my iPhone (occasionally paired to a Polar heart rate monitor) to gauge my training load. But it was an inconsistent predictor because I needed to make sure to take the reading every morning under the same conditions, etc. Given the challenges that surround our household’s morning routine, that kind of manual, tedious process for gathering HRV data never really let me get confident baseline data.
Now, the Apple Watch collects HRV data and — given how difficult it was to do it manually and get good baseline data — I was skeptical that the Apple Watch would be able to use sampling my HRV throughout the day to generate anything useful. I’m still skeptical, but my recent/current bout with Lyme disease over this past week has given me more confidence in the Apple Watch’s sampling technique to set a good baseline and show variance from that baseline.
I think this picture is pretty self-explanatory but will add that when my HRV fell to its lowest around 11/12, I could barely get out of bed, was sweating uncontrollably and could not stop shivering. By the 15th, after 36hours of doxycycline, I felt about 80% better. I am still not 100% despite a rebound in HRV.
So, HRV on the Apple Watch is good at showing when you’re REALLY sick. That seems to be all I can take away from it at this point, but at least it shows that the Watch’s sampling approach does reflect reality, even if it doesn’t yet predict or give early warning.