Mockingbirds are one of the few songbirds that naturally sing at night.   Mostly, but not exclusively, they are unmated males. They will generally stop once they have a sweetheart. They are also prone to sing when the moon is full, so I would guess light pollution may be a factor.

However, I've had them wake me up out in the country with no lights and no moon.

It seems to me that I am more likely to hear them toward the end of the mating season than at the beginning. I guess those bachelors begin to feel a little desparate!