The response of an organism to changes in day length (photoperiod). Such responses enable organisms to exploit favourable conditions associated with seasonal changes in climate and vegetation, for example to produce young when food is plentiful or to produce flowers when pollinating insects are abundant. Generally, a plant must experience a particular duration of unbroken darkness for the physiological switch from, say, nonflowering to flowering to take place. This is the critical night length, which varies between species and with latitude in the same species. Some actions are triggered when the night length falls below the critical threshold (long-day plants), whereas others are prompted when night length exceeds a critical threshold (short-day plants). Exposure to just a single requisite night length can trigger flowering in some species, whereas others require several successive exposures before flowering ensues. Moreover, some need an additional environmental stimulus, such as a period of cold, before the physiological switch is made. Plants have various photoreceptor molecules that are sensitive to light, including phytochromes and cryptochromes. It is known that in certain species these photoreceptor molecules interact with the daily fluctuations in levels of proteins responsible for the plant’s innate circadian rhythm (see biological clock) and bring about photoperiodic changes in cell metabolism. In animals, photoperiodic control of breeding is controlled by melatonin. See day-neutral plant; long-day plant; short-day plant. See also dark period.