The battery optimization feature restricts background data usage when you leave your phone unattended for a prolonged period of time, helping conserve battery life.
When you begin using your phone again, the battery optimization feature is automatically disabled.
In battery optimization mode, you may not receive alarms from some apps that have not been updated to the latest OS version.
Some notifications may not be displayed as well.
To turn on notifications permanently for select apps, go to 'Settings> Battery and power saving> Battery usage record> Optimization exception apps.'