Wearable self sufficiency

Hello, if I use a wearable, do I still need to keep the phone monitoring me directly? Or can it just (as I hope) use the data coming from the wearable in order to track my sleep phases (via, for ex, ECG)?

Because I’d like to wake up in the most comfortable phase of my hypnogram without keeping the phone too close and just wearing the device on my wrist

It will be kinda disappointing if the integrated wearable is used just as a vibrational alarm and not as a primary source of data