Garmin's owner manual for the Vivosmart 5 says that is uses HRV to calculate Stess Level. If it is measuring HRV, why can't it display it? https://www8.garmin.com/manuals/webhelp/GUID-8674F17E-62B2-48DE-927A-251611664658/EN-US/GUID-9282196F-D969-404D-B678-F48A13D8D0CB.html