Apple Watch HRV: SDNN vs RMSSD Explained

My Bio Age reads Apple Watch heart rate variability from Apple Health as SDNN, measured in milliseconds. If another app reports RMSSD, the two numbers describe different calculations. A lower number in one app does not, by itself, mean your recovery has become worse. Check what each app measures before comparing the results.

What does Apple Watch measure?

Heart rate variability describes variation in the time between heartbeats. Apple's HealthKit documentation identifies its HRV measurement as the standard deviation of normal heartbeat intervals, or SDNN. Apple Watch records these samples automatically. This is different from your heart rate, which is expressed in beats per minute.

Apple’s Series 12 announcement adds a Recovery HRV view. This guide describes the HealthKit SDNN measurement used by My Bio Age. See our Series 12 launch review for the new sensing features and their limits for third-party apps.

SDNN and RMSSD answer different questions

• SDNN describes how spread out the normal heartbeat intervals are within a recording.
• RMSSD starts with differences between successive intervals, squares them, averages them, and takes the square root.
• Both can use milliseconds. Sharing a unit does not make the numbers interchangeable.

Recording length matters too. In a study of 3,387 adults, researchers compared short ECG segments with longer recordings and found that agreement depended on both the duration and the HRV metric. The study does not validate an Apple Watch recovery score. It illustrates why the method belongs next to the number whenever you interpret HRV.

Why two health apps can disagree

Imagine that one app shows today's most recent sample while another shows the median of several samples. Those are different summaries, even if both start with SDNN. Now add a different date boundary or an RMSSD calculation and a direct comparison becomes less useful. Treat this as a checklist for reading an app's explanation, rather than assuming every app uses the same method.

1. Check whether the label says SDNN, RMSSD, or a proprietary score.
2. Check whether it represents a single sample, a daily summary, or a longer trend.
3. Look for changes in the source device or how often you wore it.
4. Compare the same app and measurement over time before comparing products.

How My Bio Age uses HRV

My Bio Age reads HRV SDNN from Apple Health. It groups available samples into daily windows and uses their median for its HRV history. That summary may differ from the value you see in another app, and it is not restricted to samples recorded during sleep. HRV also contributes to wellness calculations alongside other available signals.

Open your HRV trend alongside sleep and recovery rather than treating one unusually high or low reading as a diagnosis. If the graph is empty, follow our missing Apple Health data checklist before drawing conclusions about your health.

Can I convert SDNN into RMSSD?

There is no fixed multiplier that turns a reported SDNN value into RMSSD. The calculations use different properties of the underlying intervals. Keep the original labels when you compare or discuss your results, and avoid using a general online HRV chart as a target unless its measurement method matches.

For context on the app's estimates, read our Methods & Sources or explore the Recovery & Strain dashboard. My Bio Age supports wellness and fitness reflection; it does not diagnose heart conditions or determine whether exercise is medically safe.

Review your Apple Health SDNN history alongside sleep and recovery, with the measurement kept in context. Download My Bio Age free on the App Store. The iPhone app is completely free, with no in-app purchases, subscription or account required. Your health data is processed privately on your iPhone.