HKQuantityTypeIdentifierHeartRateVariabilitySDNN — what SDNN is in your Apple Health export
Apple Health records heart-rate variability under one identifier, HKQuantityTypeIdentifierHeartRateVariabilitySDNN, and the last four letters decide whether a number you read from it means anything next to another device.
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SDNN is the method, not a brand
SDNN is the standard deviation of the intervals between heartbeats over a window — for Apple, a roughly one-to-five-minute window the Watch chooses, usually while you are still. It is a real, defined measure of variability, and it is the only HRV Apple exports. The identifier names the method in its own text: the SDNN suffix is not decoration.
Why it will not match your Oura or Whoop number
Oura, Whoop and most nightly-readiness apps report RMSSD — the root mean square of successive differences — measured overnight. SDNN and RMSSD are computed from the same beat intervals but answer different questions, over different windows, and there is no formula that converts one to the other. A 45 ms SDNN from your Watch and a 45 ms RMSSD from your ring are not the same 45 ms, and charting them on one axis is the mistake that makes HRV look broken. It is why our parser refuses to convert between them.
What you get here
Drop your export and this page finds every SDNN sample, its millisecond value, and the timestamp — read on your device, nothing uploaded. It does not turn the number into a readiness score, because SDNN alone is not one; it shows you the measurement Apple actually stored.
This identifier is one of dozens. The Apple Health data dictionary lists every identifier this parser reads, with its canonical type and unit.