Actually, IIRC, the thing that caused the fall of civilization in Azimov's original short story "Nightfall" was not the darkness per se. It was the appearance of stars. The planet of Kalgash was much closer to the center of the galaxy (which we now know could never support similar life to ouselves) and the sky was far more filled with stars, many of which were larger than we see. This brought home to the population that they were "not alone" in the universe and supposedly that's what drove them mad.