A quantity sample type that measures the standard deviation of heartbeat intervals.
- iOS 11.0+
- watchOS 4.0+
Heart rate variability (HRV) is calculated by measuring the variation between individual heartbeats. While there are multiple ways of computing HRV, HealthKit uses SDNN heart rate variability, which is calculated using the standard deviation of the inter-beat (RR) intervals between normal heartbeats (typically measured in milliseconds).