I just discovered the new setting on ios Garmin Connect App: Garmin Devices / Fenix 5x Plus / Activity Tracking / Pulse Ox / Sleep Tracking.
You can enable also Sleep Pulse Ox from the watch menu, on Pulse Oximeter Option.
Software Version: 5.57(b82b4d4)
GPS: 2.30
Wi-Fi: 2.50
CIQ: 3.0.4
BMX: 0.4.3
WHR: 20.03.32
ANT/BLE/BT: 3.11
Sensor Hub: 2.21 ciq.forums.garmin.com/.../1422747.png