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  "title" : "heartRateVariabilitySDNN"
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# heartRateVariabilitySDNN

A quantity sample type that measures the standard deviation of heartbeat intervals.

```
static let heartRateVariabilitySDNN: HKQuantityTypeIdentifier
```

## Discussion

These samples use time units (described in [`HKUnit`](/documentation/HealthKit/HKUnit)) and measure discrete values (described in [`HKQuantityAggregationStyle`](/documentation/HealthKit/HKQuantityAggregationStyle)).

HealthKit calculates the Heart rate variability (HRV) by measuring the variation between individual heartbeats. While there are multiple ways of computing HRV, HealthKit uses SDNN heart rate variability, which uses the standard deviation of the inter-beat (RR) intervals between normal heartbeats (typically measured in milliseconds). The system automatically records samples on Apple Watch.

## Topics

### Metadata

[`HKMetadataKeyAlgorithmVersion`](/documentation/HealthKit/HKMetadataKeyAlgorithmVersion)

A key that indicates the version number of the algorithm used to calculate the sample’s value.


---

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