Sky brightness, or darkness as we here prefer, is probably one of the most important factor in visual or instrumental observation of celestial objects. The lower the sky brightness number, the better the contrast we can have for the target object.
Here is a simple chart that shows two weeks of continuous monitoring of the sky. The top numbers are when it was night time. These numbers are in calibrated magnitude scale.
The dips there are the presence of clouds which made the number go a bit higher momentarily during the nightime. Remember, magnistude scale is reversed.