Apple Watch Series 12 reads heart rate every five seconds

Apple announced the Watch Series 12 and Watch Ultra 4 on 9 September 2026, both carrying new optical and electrical heart sensors. The watches sample heart rate every five seconds through the day, where earlier models took a reading once every several minutes, and measure heart rate variability as often as every five minutes — 24 times more frequently than the previous generation.

The extra sampling feeds a daily Readiness score: a single number from 0 to 10, paired with one of four instructions — Recover, Pace Yourself, Ready, or Go For It. watchOS 27, which carries the feature, becomes available on 14 September 2026 and requires an iPhone 11 or later running iOS 27.

Apple’s announcement claims measurement accuracy, not diagnosis. It asserts no FDA clearance or authorisation for the new sensing, which keeps it on the wellness side of the line the agency draws around medical devices. The distinction governs what the readings can be used for: a heart rate sampled every five seconds is a self-tracking instrument, and a denser record of an organ most people never observe directly, but it is not a clinical measurement until a regulator says it is.