Many plants synchronize their growth stages to the seasons to ensure flowers mature and set seed when conditions are best for pollination and seed dispersal. Day length also influences dormancy in some plants and ensures that winter buds form before it gets too cold.

Photoperiod is the length of time during which plants receive sunlight and conduct photosynthesis. Plants can sense the length of time in a day and night. Plants that respond to photoperiod by altering growth and maturation exhibit the phenomenon "photoperiodism."

In many species, the perceived day length is an environmental cue that lets the plant know when it is time to advance to the next growth phase. When the photoperiod is shorter, they perceive a shorter day length that signals the approaching change in season. Short day plants, for example, respond to shorter day length by triggering flower production and maturation. Growers can take advantage of a plant’s response to photoperiod to shorten cropping time and help schedule cropping in greenhouses.

When it comes to light deprivation and plant light cycles, a 12-hour photoperiod would mean 12 hours of light exposure and 12 hours of darkness. An 8-hour photoperiod corresponds to 8 hours of light exposure and 16 hours of darkness. 100% light proofing during the dark periods is needed!

There is good evidence that the time in darkness is more important than the time in daylight to plants that respond to photoperiods. That is, lengthening of darkness is more critical to the plants than the byproduct of a shorted period of light exposure. Interruptions of light will disrupt the photoperiod effect so it is vital to ensure that plants are completely deprived of light for an uninterrupted period of darkness.

Photoperiod changes naturally with the seasons and daylight intervals (DLI) change. In the northern hemisphere, December 21 is the shortest day of the year. Day length will increase from December 21 to June 21, the longest day of the year, decreasing thereafter. Increased latitude will have a larger effect on seasonal day length fluctuation. At 30° latitude (New Orleans), day length will range from 11 hours to about 14.5 house, while at 45° latitude (Minneapolis), day length will range from 9.5 hours to 16 hours.

In some species of plants, certain varieties of the species will exhibit different responses to photoperiod. Specific photoperiod lengths to trigger maturation may differ from average estimates.