RMSSD, SDNN and pNN50 explained
All three start from the same thing: the list of intervals between your beats. They just summarize it differently.
The three measures
- RMSSD — the root mean square of successive differences between beats. It reflects fast, beat-to-beat changes and is the most common short-recording HRV figure.
- SDNN — the standard deviation of all the intervals. It captures overall variability, fast and slow.
- pNN50 — the percentage of successive intervals that differ by more than 50 milliseconds.
Which one to watch
For short daily readings, RMSSD is the best single trend to follow, which is why the app shows it on every reading. SDNN and pNN50 add detail when you want it.
FAQ
- Why does Apple Health show SDNN?
- Apple Health stores HRV as SDNN. When you turn on Apple Health, the app writes each reading's SDNN there where the recording supports one.
Your pulse in 45 seconds
Heart Rate & Health - Novaz reads your heart rate from your iPhone camera, with HRV, stress, energy and a Heart Score. No account — your data stays on your phone. A wellness app, not a medical device.
Download on the App Store