To set your screen's brightness any time, open quick settings and slide 
Your phone uses Adaptive brightness to automatically adjust your screen to the light around you. When the feature is on, you can still fine tune the brightness in quick settings.
To turn Adaptive brightness on or off, go to Settings > Display > Adaptive brightness.
When you view HDR content – like an Ultra HDR photo in Google Photos, an HDR video on YouTube, or an HDR feed on Instagram – your phone automatically sets brightness to its highest limit to make the highlights pop.
You can limit this brightness to a more comfortable level or turn off the automatic adjustment, forcing HDR photos and videos to play at a normal brightness level.
Adjust settings:
Adaptive brightness uses the ambient light sensor to adjust your display brightness automatically. When you change the brightness slider, your phone learns your preferred levels for various lighting environments. Ideally, you’ll need to use the slider less and less.
If Adaptive brightness stops working as desired, you can reset it and then retrain it.

If you prefer to manually adjust brightness, go to Settings > Display > Adaptive brightness and turn Use adaptive brightness off 
If Battery saver is on, the screen dims when the battery gets low.