This type of flatlined graph usually indicates that the app either couldn’t access or lost access to your device’s audio chip during the night.
Several things can interfere with the audio chip, such as Bluetooth connections (e.g., speakers, CarPlay), phone calls, other apps using the microphone or speaker, or background features that may access audio input.
If the issue happens consistently, we recommend trying a process of elimination to identify the cause.
For example, try disabling Bluetooth, avoid playing music or using other apps that may access the microphone or speaker, and make sure no other audio-related features are active overnight.
This is what the graph can look like if you experience the issue above.