The app advises you to turn your phone over so the screen is facing the bed. The screen turns off but the game is still running. It also advises you not to place your phone under a pillow or blanket to prevent overheating.
They tell you to place the phone facedown so the screen will at least dim & not be keeping you from sleeping (hopefully). Otherwise yeah, the alternative is to get the device that can track sleep without leaving the phone screen on.
Wow, I didn’t think I paid that much for mine. I preordered it when it came out, but I thought I was paying closer to $50. Not cheap either, but still not $75