my music is not syncing to my watch when on charger at night...

I was having the same problem and found there was a disconnect with my appleid on just my watch. This was preventing music from syncing at night when my watch was charging.

try these steps:

1. open Settings from your Aplle Watch(you have to do this on your watch, not the watch app on iPhone)
2. Select ‘Music’
3. Are you getting prompted to enter your AppleID password?
4. if so, enter your password (digital touch is the most reliable way, but you can also use the keyboard option from an iOS device
5. once your password is entered place your watch on the charger and open the Watch app from your iPhone.
6. Select Music option to see playlist and verify the sync is occurring.

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M1 Mac mini, macOS 11.2.2; Watch, series 5 GPS/cellular, watchOS 7.3.1; iPad Pro 2nd Gen, iPadOS 14.4; iPad Air 2, iPadOS 14.4; iPhone 12 Pro, iOS 14.4; TV 4th Gen, tvOS 14.;  HomePods, audioOS 14.4 and Airport Express w/ Airplay 2