Photosynthesis occurs in the presence of light, primarily during daylight hours when sunlight is available. It takes place in the chloroplasts of plant cells, specifically in the chlorophyll-containing thylakoid membranes. The process has two main stages:

1. Light-dependent reactions: These occur only when light is available. Chlorophyll captures light energy, which is used to produce ATP and NADPH, essential energy carriers.
2. Light-independent reactions (Calvin Cycle): These reactions do not require light directly and can occur as long as ATP and NADPH produced by the light reactions are available.

Although photosynthesis predominantly happens during the day, artificial lighting with the right wavelengths (blue and red) can also drive photosynthesis at night. However, plants normally rely on natural sunlight for this process. In summary, photosynthesis mainly occurs when light is available, typically during the day, because the initial light-dependent reactions require light energy to drive the process.