It’s actually quite possible to use the Wear OS wearable without a live BT connection with Sleep as Android.
All data will be kept on the watch until connection is established again, and then sent at once.
However it’s not straightforward to achieve this - the flow would need to be like this:
- start sleep tracking, let the watch connect to Sleep as Android
- now you can turn off bluetooth
- at some point during the night, you need to turn on bluetooth again (using perhaps some automation app like Tasker)
If you don’t turn on bluetooth again during the night, you won’t get any alarm on your wearable. If you don’t care about this, you can turn on bluetooth when your alarm rings on the phone (but you’ll need to be careful not to dismiss the alarm before the data have transferred over, which can take some time…)
So all in all, this scenario is possible, but not really supported at this moment.