Doze and App Standby For some models, Doze and App Standby has been added to help reduce battery consumption (Android 6 and higher mostly). From app settings, you can whitelist apps to prevent it from being affected by Doze or App standby. Some apps are already exempted such as critical system apps. If you are not receiving notifications from a particular app, like a third party alarm clock, you can add the application to a whitelist that will ignore the battery optimizations. As a workaround to ensure notifications are received : 1) Go to Settings > General tab (if applicable) > Battery & power saving > Battery usage > Ignore optimizations. 2) Tap the app to ignore battery optimizations.
Second possibility how to find it: 1) From a Home screen, navigate: Apps > Settings > Battery. 2) Tap the Menu icon (three dots located in the upper-right). 3) Tap Battery optimization. Note that Apps/services unable to be optimized appear grayed out. 4) Tap the "Not optimized" dropdown menu icon (located at the top). 5) Tap All apps. 6) Tap the appropriate app - Should I Answer, then tap "Don't optimize", then tap Done