The word "photoperiod" is correct and usable in written English
Generally, you can use it when you are referring to the amount of time plants and animals experience light in a given 24-hour period. For example, you could say, "Some flowering plants rely on a long photoperiod to thrive."
In every case, the critical photoperiod had declined over time.
Those of major importance in contributing to the onset of dormancy include changes in temperature and photoperiod and the availability of food, water, oxygen, and carbon dioxide.
Fluctuations in the major daily variables light and temperature can induce rhythmical changes in the metabolic activity of an organism; annual fluctuations in temperature and photoperiod can influence the availability of food and water.
The metabolism of poikilothermous animals is most influenced by the environmental variables of temperature, nutrition, and photoperiod.
As in the case of vernalization, photoperiod undoubtedly affects the metabolism of the known plant hormones, and so influences many other developmental responses apart from flowering.
Budbreak in certain trees is controlled by photoperiod, growth resuming in the lengthening days of spring; light-perceptive organs are probably the young leaves inside the bud scales.
A finding that is directly related to the human condition is that putting the animals on different photoperiods [day and night cycles] changes the neurotransmitter identity in the hypothalamus [a part of the brain] and this changes the animal's behaviour.
Other plants either require days of intermediate length for flowering or respond to a sequence of different photoperiods.
Newborn mice maintained during the first week of life in regular, natural photoperiods will, on reaching maturity, ovulate regularly.
In temperate zones most species breed in the spring, when water temperatures are rising and day lengths (photoperiods) are increasing.
Because of the effects of cyclical photoperiods, spontaneous ovulation occurs about the same time of day or night in all members of species intensively studied thus far.
