Plants use photosynthesis to make sugar, which serves as an energy source, and is used to help plant growth. This process occurs only during the daytime, as plants need sunlight to carry out photosynthesis. Excess sugar produced is stored in the plant until needed, such as at night or during the winter. During those times when the plant is unable to use photosynthesis for energy, those sugar stores help keep the plant healthy and allow it to continue growing.