Found this study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9412437/ where was compared data accuracy of Forerunner 245 with other devices.
I can't understand how they get HRV data from Garmin's app from Forerunner 245?
The Garmin app was also used to extract measures of heart rate and heart rate variability for a 3-min period as close as practicable to the start of each sleep period, i.e., ~30–60 min prior to lights out.