Yes, neon tetras need darkness. Neon tetras need darkness to rest, and resting is very important for any living being. The darker their environment is at night, the better their quality of sleep will be. Neon tetras require 12-14 hours of aquarium light during the day, every day, and then complete darkness at night.
Also, do not be alarmed if some of your neon tetras lose their color at night. Neon tetras typically lose their color at night and during extended periods of darkness.