In most latitudes there are seasonal changes in the length of the photoperiod (i.e. changes in the length of the day). Only at the equator are the day lengths approximately LD 12:12 all through the year. There is therefore an advantage for organisms to be able to anticipate seasonal changes, so that environmental conditions are most suitable, e.g. breeding in spring or summer to take advantage of warmer temperatures, producing flowers at the right time of year to attract pollinators, change fur colour to camouflage against predators or migrate to avoid the harsher conditions of winter.
Most seasonal events are triggered by a photoperiod of a certain length. This specific photoperiod length is referred to as the critical day length or the critical photoperiod (although more properly the critical photoperiod is the 24 hour LD ratio at which half the population switches from one state to another e.g. non-flowering to flowering). The length of the critical photoperiod varies not only between species but also between the same species at different latitudes. For example the butterfly Acronycta rumicis has a critical day length of 15 hours at 45°N, but a critical day length of 18 hours at 50°N. Most critical photoperiods are between 10 and 14 hours of light (Binkley 1997).
With some organisms an action will be triggered when the photoperiod falls below the critical photoperiod (short day), whilst in others an action will not occur until the photoperiod had passed the length of the critical photoperiod (long day). Hence these two terms can be used to categorise organisms by photoperiodicity – short day plants will flower when the photoperiod falls below the critical day length; long day plants will flower then the photoperiod is longer than the critical day length. These effects can be replicated in the laboratory, making it possible to determine the exact length of a photoperiod required to initiate a certain rhythm.
Some organisms have rhythms that are not affected by photoperiod. Often other factors will have an effect instead e.g. temperature.
Reproduction in many species occurs at specific times of the year. Plants produce pollen at the times of year when pollinators will be active, whilst animals often reproduce in the spring and summer to take advantage of the warmer temperatures. Often reproduction is triggered by a critical photoperiod. In many male animals, testis size is affected by photoperiod. In both hamsters and some birds for example the testes are small in short days (i.e. in winter) but grow dramatically in long days (Anand 2002). In the case of hamsters the critical photoperiod is 12.5 hours. When the photoperiod drops below this, the testes reduce in size and stop producing sperm. When the photoperiod is longer than 12.5 hours, the testes enlarge (testicular recrudescence). Female hamsters have a similar critical day length so that the breeding cycle of both sexes coincide.
The photoperiodic control of reproduction may be due to the hormone melatonin. This is produced by the pineal gland. Melatonin inhibits reproduction by blocking the hormone prolactin, which is a gonad stimulating hormone. Repression of prolactin causes gonad regression.
Melatonin is produces by a photoperiodically controlled cycle. Its precursor seratonin is transformed to melatonin by a process that involved the enzyme N-acetyltransferase (NAT). NAT has a cycle that has an amplitude that is suppressed by constant light but freeruns in constant dark. Normally NAT and melatonin are produced at night.