The short answer
Apple Watch HRV is a reference value for how much the interval between heartbeats changes. HealthKit calculates it using a method called SDNN and records it in milliseconds. Rather than treating a single reading as a pass or fail, compare it with your own history measured under similar conditions.
Key points
- Heart rate is "beats per minute"; HRV is "how much the gaps between beats vary"
- Apple Health records HRV as SDNN, in milliseconds
- HRV is affected by age, time of day, posture, breathing, sleep, exercise, alcohol and how you feel
- Your own trend over weeks tells you more than one reading or someone else's number
A heartbeat is not a metronome
At 60 beats per minute, the beats do not land exactly one second apart. They come at 0.94 seconds, then 1.06, then 0.98 — the interval shifts constantly with your breathing and the body's own adjustments. HRV (Heart Rate Variability) is a way of summarising that shifting.
If heart rate is the average tempo, HRV is the fine wobble within it. Similar names, different things.
What SDNN on Apple Watch means
There are several ways to calculate HRV. Apple's HealthKit uses SDNN, which describes the spread of intervals between normal beats, and records it automatically as discrete samples. The unit is normally milliseconds (ms).
A different method, a different measurement window, body movement or how the watch sits on your wrist will all change the number for the same person. It is safer not to line up one Apple Watch reading against a figure from another brand or a laboratory measurement.
Is higher better and lower worse?
No general rule can draw a line at "healthy above X ms". HRV varies widely with age and between individuals, and shifts with time of day, posture, breathing, exercise, sleep, alcohol, medication and sudden changes in how you feel. Measurement noise and irregular beats can affect the figure too.
Keep recording with the same device, and read "how far this has moved from my own usual range" alongside your sleep, your resting heart rate and how the day actually feels. HRV alone cannot determine stress, illness or recovery.
Where to find HRV on iPhone
Open Health
Open the Health app on your iPhone.
Go to Heart from Browse
Choose Browse → Heart → Heart Rate Variability. The label and its position vary slightly between OS versions.
Switch the range
Move between day, week, month, 6 months and year, and look at the flow rather than a single point.
Check the data source
Show All Data, or the data source list, tells you when each sample was recorded and by which device.
References
- HealthKit heart rate variability (SDNN) specification — Apple Developer
- Measure your heart rate with Apple Watch — Apple サポート
- Consumer Wearable Health and Fitness Technology in Cardiovascular Medicine — Journal of the American College of Cardiology