Chronobiology is a field of biology that examines periodic phenomena in living organisms and their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek , and biology, which pertains to the study, or science, of life. The related terms chronomics and chronome have been used in some cases to describe either the molecular mechanisms involved in chronobiological phenomena or the more quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is required. Chronobiological studies include but are not limited to comparative anatomy, physiology, genetics, molecular biology and behavior of organisms within biological rhythms mechanics. Other aspects include epigenetics, development, reproduction, ecology and evolution.
Description
The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur in animals, in plants, and in microbial organisms such as fungi and protozoa. They have even been found in bacteria, especially among the cyanobacteria. The best studied rhythm in chronobiology is the circadian rhythm, a roughly 24-hour cycle shown by physiological processes in all these organisms. The term circadian comes from the Latincirca, meaning "around" and dies, "day", meaning "approximately a day." It is regulated by circadian clocks. The circadian rhythm can further be broken down into routine cycles during the 24-hour day:
Diurnal, which describes organisms active during daytime
Crepuscular, which describes animals primarily active during the dawn and dusk hours
While circadian rhythms are defined as regulated by endogenous processes, other biological cycles may be regulated by exogenous signals. In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members. The endogenous plant cycles may regulate the activity of the bacterium by controlling availability of plant-produced photosynthate. Many other important cycles are also studied, including:
Infradian rhythms, which are cycles longer than a day. Examples include circannual or annual cycles that govern migration or reproduction cycles in many plants and animals, or the human menstrual cycle.
Within each cycle, the time period during which the process is more active is called the '. When the process is less active, the cycle is in its ' or trough phase. The particular moment of highest activity is the peak or maximum; the lowest point is the nadir. How high the process gets is measured by the amplitude.
History
A circadian cycle was first observed in the 18th century in the movement of plant leaves by the French scientist Jean-Jacques d'Ortous de Mairan. In 1751 Swedish botanist and naturalistCarl Linnaeus designed a flower clock using certain species of flowering plants. By arranging the selected species in a circular pattern, he designed a clock that indicated the time of day by the flowers that were open at each given hour. For example, among members of the daisy family, he used the hawk's beard plant which opened its flowers at 6:30 am and the hawkbit which did not open its flowers until 7 am. The 1960 symposium at Cold Spring Harbor Laboratory laid the groundwork for the field of chronobiology. It was also in 1960 that Patricia DeCoursey invented the phase response curve, one of the major tools used in the field since. Franz Halberg of the University of Minnesota, who coined the wordcircadian, is widely considered the "father of American chronobiology." However, it was Colin Pittendrigh and not Halberg who was elected to lead the Society for Research in Biological Rhythms in the 1970s. Halberg wanted more emphasis on the human and medical issues while Pittendrigh had his background more in evolution and ecology. With Pittendrigh as leader, the Society members did basic research on all types of organisms, plants as well as animals. More recently it has been difficult to get funding for such research on any other organisms than mice, rats, humans and fruit flies.
Recent developments
More recently, light therapy and melatonin administration have been explored by Alfred J. Lewy, Josephine Arendt and other researchers as a means to reset animal and human circadian rhythms. Additionally, the presence of low-level light at night accelerates circadian re-entrainment of hamsters of all ages by 50%; this is thought to be related to simulation of moonlight. Humans can have a propensity to be morning people or evening people; these behavioral preferences are called chronotypes for which there are various assessment questionnaires and biological marker correlations. In the second half of 20th century, substantial contributions and formalizations have been made by Europeans such as Jürgen Aschoff and Colin Pittendrigh, who pursued different but complementary views on the phenomenon of entrainment of the circadian system by light. There is also a food-entrainable biological clock, which is not confined to the suprachiasmatic nucleus. The location of this clock has been disputed. Working with mice, however, Fuller et al. concluded that the food-entrainable clock seems to be located in the dorsomedial hypothalamus. During restricted feeding, it takes over control of such functions as activity timing, increasing the chances of the animal successfully locating food resources. In 2018 a study published in PLoS ONE showed how 73 psychometric indicators measured on Twitter Content follow a diurnal pattern.
