title: Querying for heartRateVariabilitySDNN in HealthKit?
body: <p>I notice in the Health App I receive a daily value for Heart Rate Variability.  Per the HealthKit <a href="https://developer.apple.com/documentation/healthkit/hkquantitytypeidentifier/2881127-heartratevariabilitysdnn" rel="nofollow noreferrer">docs</a> this is a sample that measures the standard deviation of heartbeat intervals.  Yet when I query for HRV in my workout app during a workout I am not getting any samples returned.  Is there a particular circumstance that must be present to return a value or is Apple just not sharing this with 3rd party developers yet (like vo2Max?).   </p>