The sleep HRV data from my Garmin differs from the display in Sonar. Just as an example see data of the last four days: 43 44 44 43 (Sonar) vs 48 48 49 47 (Garmin). In the beginning it seemd that data form Garmin were mirrored in the Sonar app, but at some point weeks ago that changed. Since there is no other source of HRV data configured and I also blocked the export of HRV data from Apple Health I have no explanation for the situation. I do record HRV data in Apple Health from a Polar H10, but the only data granted for export are BPM data. I also have the Elite HRV app on my phone, which I use to measure the morning readiness data, from which the SDNN value is shared to Apple Health, but the Elite app is not configured as a data source in Sonar. To me it looks like the final data in Sonar were averaged with some other data, but there is no configuration that would allow that.