This notification will save you from needing to constantly check your Apple Watch to see when it can be taken off the charger.
To perform these steps you will need your iPhone and a paired Apple Watch.
Make sure Sleep mode is enabled on your iPhone.
● How to Turn On Sleep Mode on iPhone [Video]
Tap to launch the Watch app from your Springboard, then select the My Watch tab at the bottom of the screen.
Choose Sleep from the list, then toggle the Charging Reminders switch to the ON position.
This feature will remind you to charge before bedtime and notify you when your Apple Watch is fully charged. The Apple Watch battery notification appears on your paired iPhone just like regular push notification.