Apple Watch reports HRV as SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , taken from short readings the watch records on its own, while most rings and bands report RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD averaged over your sleep. These are different statistics measured at different times, so the numbers are not directly comparable, even when both devices measure your heartbeats well.
Two different statistics
Both statistics start from the same thing: the time between one heartbeat and the next. They then summarise those gaps in different ways.
- SDNN is the standard deviation of all the gaps in a recording. It shows how spread out the gaps are overall. Apple’s developer documentation says HealthKit uses SDNN for heart rate variability.
- RMSSD looks at the change from each gap to the next one. It shows how much the timing jumps from one beat to the next.
A widely cited review of HRV metrics explains that RMSSD is the main time-based measure of the calming ( parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system ) branch of the nervous system. SDNN reflects both branches, the calming one and the “ fight or flight Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system ” (sympathetic) one. Because they capture different things, there is no fixed exchange rate between them. The lesson on RMSSD, SDNN and lnRMSSD shows how each one is calculated.
Different moments, different lengths
The second difference is timing. Apple says the system records HRV samples automatically on Apple Watch. On Apple Watch Series 12 and Ultra 4, Apple says Recovery HRV is sampled while you are still, so the number of readings you see depends on how active you are. Oura, WHOOP, Fitbit and Polar instead take their HRV from sleep. The HRV by device guide lists when each brand measures.
SDNN also depends on how long the recording is. The same review notes that SDNN is more accurate over 24 hours and that it is not appropriate to compare SDNN values from recordings of different lengths. It also warns that 24-hour, short-term and ultra-short values are not interchangeable. A short reading from your wrist and a 24-hour clinical SDNN from a heart monitor belong to different reference tables.
Is the Apple Watch accurate?
For the beats themselves, the evidence at rest is encouraging. In a 2018 study, 20 healthy volunteers sat for five minutes watching a relaxing video and then did five minutes of a stressful attention test. Researchers compared the beat-to-beat intervals from an Apple Watch with those from a chest strap. The time-based HRV measures did not differ significantly between the two, and both devices showed RMSSD falling under stress. About 10% of intervals were missing from the watch recordings, in short gaps. The study was small, used an older version of watchOS, and tested people sitting still.
So when an Apple Watch and another device disagree, the statistic and the timing usually explain the gap.
What to do with your own data
- Compare Apple with Apple. Look at your Apple Watch HRV trend over weeks in the Health app, not single readings.
- Use a matching reference. If you look up norms, use SDNN values from a similar recording length, not RMSSD charts.
- Keep conditions steady. Readings taken while you are still and relaxed are easier to compare with each other.
- Do not mix brands in one trend. If you also wear a ring or band, keep its numbers separate. See why Oura and WHOOP show different HRV for why even two RMSSD devices disagree.
When to talk to a doctor
HRV from a watch cannot diagnose a heart problem. Talk to a doctor if you have chest pain, fainting, shortness of breath, or a racing or uneven heartbeat, whatever your HRV shows.