Le Lune di Pompeii, the moons of Pompeii, is a special sound and light night tour given on weekends from May through October. The visit starts at the amphitheater gate and lasts about 70 minutes. Cost in 2010 was 20 euro.
A guide leads you through the excavations starting with a walk through the ancient necropolis, or city of the dead, to enter Pompeii through one of the ancient gates as a visitor would have done in Roman times. You'll see the casts of body shapes as they were buried by the lava, visit a couple of houses and a garden, walk on the ancient Roman streets, and end at the amphitheater for a re-enactment of the volcanic eruption.
Along the route, the group stops at designated points (where there's a white ball or moon in the sky) to listen to a recording. At one stop a woman dressed in ancient Pompeii style appeared and gave us an interesting look at ancient graffiti on the walls. Although parts of the tour were a little silly, it's nice to walk through the excavations at night and I did learn a few interesting things about the ancient city on our tour.
You'll need to make reservations for the night visit. There are some English language tours but you should inquire when you book as the tour I took was only in Italian. The web site (below) is in Italian but if you click on prenota you can get to the booking page in English.
While Pompeii can easily be visited as a day trip from Naples, if you're going to the night tour I'd recommend spending the night. We stayed at Hotel Diana (see my review of Hotel Diana), well located for the night tour or visiting the excavations during the day. Hotel Diana offers a Mount Vesuvius tour and is very near the national train station so it could make a good base for visiting the area. Hotel Diana booking and information