I've written a blog about this a while ago [0]. You're limited to what HealthKit allows you to query and the RR intervals are not available. With the release of

I've written a blog about this a while ago [0]. You're limited to what HealthKit allows you to query and the RR intervals are not available. With the release of WatchOS 4 however the SDNN heart rate variability can be queried. [0] https://www.vanmil.org/heart-rate-variability-and-brain-waves/ https://www.vanmil.org/heart-rate-variability-and-brain-wave...