It's mostly tracking change in body movement. It's not necessarily better than a wearable but I don't want to wear a device while asleep so that was a no-go anyway.

Another constraint was that I wanted to be able to track the left and right side of the bed independently so that ruled out microphone and accelerometer based solutions. To be honest I can't say that it's working super well yet and I might replace it with flexible load cells in the mattress on the left and right side of the bed.