If you reinstall the app, such as during a device change, pairing with Health (iOS) or Health Connect (Android) will be removed.
Please note that once this pairing is removed, any sleep data that has not yet been synced with Pokémon Sleep will not be able to be synced, even if you transfer your account to the new device.
After transferring your account, please perform the "Pair a Device" process again.
Only sleep data recorded after the device is paired can be synced.