Why your Apple Watch HRV does not match your Oura ring
They do not disagree. They are measuring two different things, and the usual explanations — sensor quality, wrist versus finger, calibration — are not the reason.
The actual difference, in two rows
| Apple Watch | Oura | |
|---|---|---|
| Statistic | SDNN | RMSSD |
| Window | Short spot checks — background readings and Breathe sessions | Averaged across the sleep period |
| Time of day | Mostly while awake and still | Overnight |
Two variables differ at once. Even if both devices reported the same statistic, the windows would produce different numbers; even if the windows matched, the statistics would.
Why the sensor explanation is the wrong one
It is the intuitive answer — a finger has better perfusion than a wrist, an optical sensor is noisier when you move — and those effects are real. They are also much smaller than the gap people are asking about, and they cannot explain the most obvious feature of the data: the gap has a consistent sign and rough magnitude for a given person, night after night.
A noise explanation predicts scatter. What you see is an offset. An offset is what two different statistics over two different windows produce.
What the difference does not tell you
Because there is no conversion (why), the size of the gap between your two devices is not informative on its own. It is not a measure of stress, or of recovery, or of either device's accuracy. It is an artefact of comparing two statistics that were never meant to be compared.
What to do with two devices
- Keep two baselines. One per device, per statistic. Each device's numbers are internally consistent over time, which is what makes them useful.
- Compare trends, never levels. "Both are below their own average this week" is a real signal. "The ring says 70 and the watch says 45" is not one.
- Expect them to diverge on some nights and agree on others. They respond to overlapping but different physiology, so correlation is real but partial.
- Do not average them. The mean of an SDNN and an RMSSD is not a quantity.
The same problem, other pairs
WHOOP reports RMSSD but over the slow-wave-sleep window specifically, not the whole night — so a WHOOP-versus-Oura comparison has the same shape with a smaller gap. Garmin and Polar both report nightly-average RMSSD, which makes them the most directly comparable pair of the six. The full table is on which HRV metric each device reports.