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So the documentation here only says: A quantity sample type that measures the standard deviation of heartbeat intervals. And below: Discussion These samples use time units (described in HKUnit) and measure discrete values (described in HKStatisticsQuery). 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). There is no details on how many heartbeat intervals are used to calculate the SDNN and how to modify it. What if I want to calculate the SDNN on the last 30 intervals? Or the last 10? or the last 100? Or maybe I just want to calculate it on a timeframe, like 60 seconds, regardless of there were 50 or 120 heartbeats.

Point 27.4 of the App Store Review Guidelines reads: "Apps may not use or disclose to third parties user data gathered from the HealthKit or CareKit APIs or from health-related human subject research for advertising or other use-based data mining purposes other than improving health, or for the purpose of health research" Now, when the app will be ready for release I only intend to read the data and process it directly on the phone. However, for the purpose of testing I need to extract the data, get it on some excel / csv file and analize it. I initially thought to send the data to Firebase, but was told this might breach 27.4. I only intend to have this feature in the beta I'll release on Test Flight for myself, but still Apple might refuse it. What alternatives do I have? Can I save the data locally on my watch? Might be quite heavy (monitoring up to 8 hours) How do I get it to a computer then? Can I send the data to iCloud? I won't be developing the app first hand, I'm hiring someone so I need to understand what is feasable. Thank you for your help.