There are also wristbands with accelerometers used for sleep tracking. That might be better than strapping phone to yourself, plus they can measure heartbeat. For polysomnography do you also want to measure respiration? That is yet more hardware, but EEG seems like the big step past stuff like Fitbit, which is supposedly not very good. I am a bit interested in this stuff though haven’t tried it. I don’t think I’d bother with a phone app.