We've run an experiment with PSG, Oura, Apple Watch, and Phillips Respironics devices to understand the sensitivity of all these devices. We validated that the Oura and the Apple Watch have very accurate sensors for measuring HRV compared to a clinical grade ECG (R2 is about .80 when comparing the HRV sensors of Apple Watch and Oura to clinical sensors), but that the algorithms for measuring sleep quality have some room for improvement.
When people fall asleep their HRV decreases and in deep sleep the HRV decreases as well. So a decrease in HRV during sleep is indicative of sleep quality since deep sleep is more regenerative
So generally, yes, I would say that this is the case.