Lights out on La Palma

The island of La Palma is a very good location for visual astronomy and astrophotography. The very famous Roque de los Muchachos Observatory is located here and the seeing statistics show that this is the 2nd best location in the northern hemisphere, after Mauna Kea Observatory in Hawaii.
La Palma has also the so called Sky Law which covers aviation routes, atmospheric pollution, electromagnetic radiation and crucially, exterior lighting. A lot of this is achieved simply by using street lights which send all the light downwards.

Despite these legislations there is still a bit of light polution (of course) and I was a bit surprised to notice that they didn’t automatically turn down the lights after a certain time. (which you see happening in France a lot for instance, where street lights are turned of in certain villages after 23:00)
However, when photographing I did notice something surprising; I could tell from the raw image if it was taken before or after 0:00! The difference was huge, even though I could not visually notice something happening. All lights I could see would still be on for instance. But the difference was huge, as you can see in this image:

On the left you see the frame taken right before 0:00. You can clearly see a lot of red in this image from the light polution. On the right you see the frame taken right after 0:00. ‘Something’ happened making the sky darker as you see a nice black frame all of a sudden!
What could the reason be?

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