Apple Watch Series 12 introduces what Apple calls the Health Sensing System: new optical and electrical heart sensors that measure heart rate every 5 seconds all day and heart rate variability every 5 minutes, which Apple says is 24 times more often than before.
The same system is in the Apple Watch Ultra 4, and it is the foundation for the new Readiness score and for several features in the redesigned Health app arriving with iOS 27. Our Series 12 preview covers what was expected before the event; this is what shipped.
What the new heart sensors do
Apple pairs the new optical and electrical heart sensors with larger, power-efficient green LEDs, the light source optical heart rate sensing relies on. Despite the higher sampling rate, Apple still rates Series 12 at up to 24 hours of battery life.
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Heart rate is measured every 5 seconds throughout the day, not only during workouts. Apple has not published the sampling rate of previous models, so the exact change for heart rate is unclear, while the HRV increase is spelled out at 24 times.
| Measurement | Series 12 and Ultra 4 | Apple’s note |
|---|---|---|
| Heart rate | Every 5 seconds, all day | New optical and electrical sensors |
| HRV | Every 5 minutes | 24 times more often than before |
| Recovery HRV | Daily | Tracks stress and recovery; compared with your baseline overnight |
| Overall HRV | Ongoing | Shown in the new daytime vitals view |
| Steps | New Steps complication | Apple claims the most accurate step tracking of any smartwatch |
Two kinds of HRV, and a daytime vitals view
HRV now comes in two variants. Recovery HRV is meant to reflect daily stress and recovery, and overall HRV is the broader figure. Overnight, the watch compares Recovery HRV with your personal baseline.
There is also a new daytime vitals view, so vitals are no longer limited to overnight readings. Apple has not published which additional vitals the daytime view includes beyond the heart data described in its release.
Apple’s accuracy claims
Apple says a study of more than 1,000 participants conducted in July and August 2026 found Apple Watch to have the most accurate heart rate sensing in a wearable versus leading competitors. Apple has not published the study’s methodology or named the competitors.
Apple makes a parallel claim for step counting, calling it the most accurate step tracking of any smartwatch, and adds a new Steps complication for watch faces.
Where the data ends up
On the watch, the sensors feed the new Readiness score, a 0 to 10 daily rating built from recent activity, training load, vitals and sleep. In the new Health app that ships with iOS 27 on September 14, resting heart rate and HRV from Apple Watch also contribute to Health Age, which compares your metrics against your chronological age.
Apple notes that the Vitals app is for wellness purposes, not medical use. It has not said whether older watches gain the new sampling rates through watchOS 27, and the study claims are specific to the new hardware.
Series 12 starts at $399 and Ultra 4 at $799, both in stores on Friday, September 18. Our event recap has the rest of what Apple announced today.
