The relative lengths of night and day.

This is important to almost all animals and plants, in setting their circadian rhythm.

The photoperiod is obviously different in the summer than it is in the winter, which helps to signal to plants when to flower. This type of physiological response to photoperiod is called photoperiodism.

Most animal responses are to a combination of temperature change and one in photoperiod, which usually come hand in hand. Plants, however, will respond even when other conditions remain constant.

Some plants will only bloom if there is a long, uninterrupted night. Other plants tend to bloom when there is a short night, or a long night with a brief interruption of light.

Manipulating photoperiod can help enthusiasts produce the best, brightest flowers, or can inhibit a plant from producing flowers when undesirable.