Does a cactus need sun to survive?

Yes, Although most cactus can live in very dry environments, meaning with few water, they are still ordinary plants in the sense that they get their energy from the sun via the common photosynthesis process, which uses energy from sun's light , carbon dioxide and water, to produce chemical reactions needed to convert carbon into organic molecules, produce ATP (Adenosine triphosfate, a kind of cells fuel or energy storage) and releasing oxygen as a waste product. Cactus, slow growing, but still a plant.