SDNN vs RMSSD

The two competing HRV statistics: Apple historically reported SDNN while most of the wearable industry (and most users) use RMSSD, making cross-device HRV numbers incomparable — "all sorts of comparison controversy." A same-day cross-video thread from the Series 12 launch: rob-ter-horst flagged the ambiguity in his announcement analysis and asked Apple which metric the new HRV features use; dc-rainmaker's hands-on had the answer — Apple now exposes both, per-feature (health-age uses SDNN, other features RMSSD), with everything written to Apple Health where third-party apps can read the applicable type (apple-watch).

Sources: Quantified Scientist analysis, DC Rainmaker hands-on