---
title: heartRateVariabilitySDNN
framework: healthkit
role: symbol
role_heading: Type Property
path: healthkit/hkquantitytypeidentifier/heartratevariabilitysdnn
---

# heartRateVariabilitySDNN

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

## Declaration

```swift
static let heartRateVariabilitySDNN: HKQuantityTypeIdentifier
```

## Discussion

Discussion These samples use time units (described in HKUnit) and measure discrete values (described in 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](healthkit/hkmetadatakeyalgorithmversion.md)

## See Also

### Vital signs

- [heartRate](healthkit/hkquantitytypeidentifier/heartrate.md)
- [lowHeartRateEvent](healthkit/hkcategorytypeidentifier/lowheartrateevent.md)
- [highHeartRateEvent](healthkit/hkcategorytypeidentifier/highheartrateevent.md)
- [irregularHeartRhythmEvent](healthkit/hkcategorytypeidentifier/irregularheartrhythmevent.md)
- [restingHeartRate](healthkit/hkquantitytypeidentifier/restingheartrate.md)
- [heartRateRecoveryOneMinute](healthkit/hkquantitytypeidentifier/heartraterecoveryoneminute.md)
- [atrialFibrillationBurden](healthkit/hkquantitytypeidentifier/atrialfibrillationburden.md)
- [walkingHeartRateAverage](healthkit/hkquantitytypeidentifier/walkingheartrateaverage.md)
- [HKDataTypeIdentifierHeartbeatSeries](healthkit/hkdatatypeidentifierheartbeatseries.md)
- [HKElectrocardiogramType](healthkit/hkelectrocardiogramtype.md)
- [oxygenSaturation](healthkit/hkquantitytypeidentifier/oxygensaturation.md)
- [bodyTemperature](healthkit/hkquantitytypeidentifier/bodytemperature.md)
- [bloodPressure](healthkit/hkcorrelationtypeidentifier/bloodpressure.md)
- [bloodPressureSystolic](healthkit/hkquantitytypeidentifier/bloodpressuresystolic.md)
- [bloodPressureDiastolic](healthkit/hkquantitytypeidentifier/bloodpressurediastolic.md)