I’m just starting to look into this as well, and I have some early input for you. Choose an Android standalone smartwatch with a screen resolution of at least 400X400 pixels (most AMOLED Android watches use this as standard). Lower resolution screens won’t allow the setting of the alarms as the rotating circle setting is half off the screen and the page doesn’t scroll.
The app does fully load in a watch with at least 1GB RAM and 16GB storage, but I don’t know fully about the interface to the sensors. I’m pretty sure accelerator connects. but heart rate sensor integration is usually pretty tricky for 3rd party apps. If there’s interest, I’ll report back what I find.
Any particular Android watches you want me to try? Most are big and bulky for sleeping, but the ones with removable bands are a bit more flexible in bed than the ones with fixed bands with antennas in them. Right now I’m looking at installing SAA in Android 5.1 watch and a relatively new Android 7.1.1 watch. If necessary because of Android permission issues and access to sensors, I may need to stretch back and try an Android 4.4 watch, but I hope that won’t be necessary.
FYI, I am having some good results so far with the Galaxy Watch Active (my favorite because it is so small) and the Gear S3, both Tizen OS from Samsung. The app’s not on the watches, of course, but I can get the phone to trigger the vibration via Bluetooth on each of these watches during REM and am really starting to remember my dreams better in the morning as a result of the heightened awareness that happens when the trigger goes off during the dream.
As for Android watches, preliminary testing shows severe battery drain, so that could be an issue. I may need to work with one that has 600mAh or higher. Probably start with the Kospet Hope or Hope Lite.