Knowing full well that HRV is unique to each user, I didn't really mentally register the low values being given by the Forerunner 255's optical sensor were anything to be concerned about. The sensor was and is still reading in the mid 20s for my overnight average. Definitely lower than expected based on my current fitness level but again I trusted that even if the number wasn't completely accurate it would be precise in a relative sense to whatever the actual value would be if it were to be measured scientifically. This couldn't be further from the truth.
Below are two health snapshots that I took 30 minutes after waking up and before any coffee. The top measure was done with the HRM-Pro+ heartstrap on and the bottom measure was done using the optical sensor on the Forerunner 255. You can see the numbers are wildly different.
I've repeated this experiment several times and at different times of the day. This disparity would be OK if the difference between the two values were predictable. The problem is that no matter what happens when I run the health snapshot the value coming from the optical sensor is in the 20s but the heart strap (depending on time of day, etc) has been anywhere from 50 to 99ms.
I've even tried to test with a different heart strap and the Elite HRV app with the same outcome. Vastly different values coming from the optical sensor (never outside of the 20s) and the heart straps. Below you can see EliteHRV giving a 60 and Garmin Optical Sensor giving 24.
Again, I never expected the optical sensor to be as good as a heart strap but I at least expected the values to be close or predictably skewed. This is neither.
When drilling down into the Health Snapshot data I can clearly see the optical sensor struggling to even measure my heart rate whereas the heart strap sees each breath:
Just to be sure, I did a set of 3 tests in a row (6 tests in total) where I used only the optical sensor and then the HRM-Pro strap. You can see the results are some 50ms off! How is this possible?
So, I really am left to wonder if I got a bad watch and it's a factory flaw? Is it personal to my skin (though I'm pasty white and have tried the sensor on top and bottom of my wrist with the same results)? OR Is the garmin forerunner 255 optical HRV just completely trash and a worthless metric?
Would really love some help finding out either how to get better measurements from the watch or if something is wrong with my watch.
