Do you still need to place the phone on your bed/near you when you’re using a fitness tracker with the app? I guess this depends on whether the fitness tracker replaces the phone’s sensor or supplements it but the app documentation doesn’t seem to make this point clear.
If you are tracking using sonar, the phone can go 1m away on bedside table. If you are using the phone’s accelerometer then it goes on the bed. There’s a bit about the different sensors here https://docs.sleep.urbandroid.org/sleep/sensors.html
If you manage to link up a fitness tracker then the phone shouldn’t need to go on the bed as the app is using the tracker data. I tried to link up my fitbit but it won’t.