Cacti (and all plants) do need a dark cycle! 24 hour light will work okay at first, but eventually it catches up and the plant will suffer. An 18/6 light cycle would be a good compromise.

As AlbertKLloyd mentioned, check out CAM metabolism, which is a little different than metabolism in most other plants. Cacti open their stoma for gas exchange at night, and actually absorb CO2 and store it in a converted form within the plant, which is then used for photosynthetic production during daylight hours. Pretty cool, part of the way cacti have adapted to arid climates and conserve their water (stoma are closed during the day to prevent water loss). CAM also helps adapt them to hot temperatures and plenty of sun.