The rising and setting of the sun provides a regular transition between two starkly different environmental conditions for photosynthetic organisms such as plants. Since photosynthetic organisms rely on light as both an environmental signal and an energy source it is beneficial for plants to respond to and utilize light in a time-dependent manner. This timing information is provided by the circadian system, a complex biological oscillator that provides an internal reference that can be used to modify plants’ responses to light signals. In this review, we will discuss how the circadian system regulates photosynthetic processes, while also exploring how photosynthetic products can themselves feed into the circadian system to optimise interactions between plants and their environment.