The amount of daytime in a day depends on the season. Summer is known for longer days, and in the winter we have longer nights. Plants can respond to the amount of light they receive.
The photoperiod is the amount of time when we give light to plants. Not all plants respond this, but there are two groups of plants that do, they are called long-day plants and short-day plants. Long-day plants flower when they get more light than dark (a longer day), and short-day plants flower when they get more dark than light (a shorter day and longer night). By controlling the amount of light the plants get you can make plants flower earlier or later than they normally would.
How do you think Sunset and Sunrise fit into the photoperiod? How would you recreate sunrise and sunset in your setup?