What it measures
Your heart does not beat like a metronome. The variation between beats reflects how your nervous system is balancing rest and stress. Apple Watch reports it as SDNN, in milliseconds.
What is a good HRV
There is no single good number: HRV falls with age and differs a lot between people. Compare yourself with your own normal, not with someone else's.
Reading it well
Use overnight readings, which are steadier than daytime ones, and watch the trend over a week rather than one value.
How Life OS uses it
Life OS reads last night's HRV and resting heart rate from Apple Health and compares them with your own baseline to build your readiness score.
Questions people ask
What is a normal HRV?
It varies widely by age and person. The useful comparison is with your own recent average.
Why is my HRV low today?
Common causes are short or poor sleep, alcohol, illness, stress or hard training the day before.