RMSSD vs SDNN — which HRV does Apple Watch use?
Short answer: RMSSD measures beat-to-beat variation and mainly reflects parasympathetic (recovery) activity, which makes it the standard for daily recovery tracking. SDNN measures overall variation across a recording. Apple Health has long reported HRV as SDNN, so Apple Watch numbers aren't directly comparable with WHOOP or Oura's RMSSD.
The two metrics
| RMSSD | SDNN | |
|---|---|---|
| Stands for | Root mean square of successive differences | Standard deviation of NN (normal-to-normal) intervals |
| Captures | Beat-to-beat changes | All variation over the recording |
| Mainly reflects | Parasympathetic (vagal) activity | Parasympathetic + sympathetic + slower rhythms |
| Best for | Short daily readings, recovery | 24-hour clinical recordings |
| Used by | WHOOP, Oura, Garmin, most sports research | Apple Health |
Why Apple uses SDNN
SDNN is a long-standing clinical measure from the 1996 HRV standards. Apple Watch takes short readings throughout the day and night and stores them in Apple Health as SDNN in milliseconds.
Why your numbers don’t match other wearables
- Different formula: SDNN and RMSSD give different values from the same heartbeats.
- Different window: a 1-minute spot reading isn’t the same as a whole-night average.
- Different timing: a reading taken during deep sleep is often higher than one at 3 pm.
Rule of thumb: compare each device only with itself.
How Pulse Coach handles it
Where Apple Health also records RMSSD and has at least 7 days of it, Pulse Coach uses RMSSD for your recovery score. Otherwise it uses SDNN. Either way, today’s value is compared only with your own 60-day baseline from the same metric. See How it works.
Questions people also ask
Is SDNN or RMSSD better for recovery?
For short daily readings, RMSSD is the more established recovery marker because it tracks parasympathetic activity and is less affected by breathing rate. SDNN still trends in the same direction for most people, so it remains useful compared with itself.
Can I convert SDNN to RMSSD?
Not reliably for one person. They're correlated across populations, but the ratio changes with heart rate and recording length.
Sources
Not medical advice. This article is general information about wellness metrics. It is not a diagnosis or a substitute for professional care. If a reading worries you, or you feel unwell, talk to a doctor.