To set your screen's brightness, open quick settings and slide 
Your phone automatically adjusts your screen to the light around you, with Adaptive brightness. When the feature is on, you can still fine tune the brightness in quick settings. To turn this feature off:

If the brightness slider 
Use extra brightness when you:
The bump in brightness uses a lot of battery power (and can cause eye strain in darker environments), so when you don’t need extra brightness, turn it off.
To adjust this setting:


You can also make the screen extra dim if the slider in quick settings doesn't go low enough.