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The minimum brightness on many phones is too bright for use in the middle of the night or when you want to fall asleep soon.
The app screen filter enables you to overlay a filter on your screen that reduces the brightness of the screen even more, however it is still important that you use the lowest brightness setting of your phone as well.
You can download screen filter here: Screen Filter - Android-apps op Google Play
I myself use the screen filter on 22.8 percent just before I go to sleep, but this can be different on any phone.
If you made the screen to dark to read, press the home button, swipe down from top the bottom to show the notification screen and click on a notification (I know you can't see them but guess )
if you click the screen filter notification the brightness will automatically go up again.
No worries the app will also prompt you to confirm very low brightness settings. and has a live preview of the screen brightness before you enable the screen filter.