Even plants need a break from the business of growing. It's during dark periods that plants turn from producing oxygen to producing carbon dioxide, an important part of their growing process. Without a good balance between resting periods an growing periods, many plants won't bloom, set buds or produce fruit.
Some plants are so dependent on dark periods that they need a changing schedule of light and dark in order to trigger flowering. This light/dark schedule is independent of seasonal temperature and weather. Popular examples include chrysanthemums, poinsettia, and christmas cactus. Other examples include hibiscus and several popular vegetables like potatoes, lettuce, and radishes.
