* Sunlight is essential: Photosynthesis is the process where plants use sunlight, carbon dioxide, and water to create food (sugar) and oxygen. Without sunlight, this process cannot occur.
* Nighttime is for respiration: At night, plants switch to respiration, where they break down the sugar they made during the day to fuel their growth and other processes.
So, plants need sunlight to photosynthesize, which is why they primarily do it during the day.