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 development, reproduction, ecology and evolution.
The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.), and (b) in plants (leaf movements, photosynthetic reactions, etc.). The most important rhythm in chronobiology is the circadian rhythm, a roughly 24 hour cycle shown by physiological processes in plants and animals. (The term circadian comes from the Latin circa, meaning "around" and dies, "day", meaning "approximately a day."). Many other important cycles are also studied, including:
A circadian cycle was initially discovered, in the 1700s, in the movement of plant leaves by the French scientist Jean-Jacques d'Ortous de Mairan (for a description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin see this page). In 1751 Swedish botanist and naturalist Carolus Linnaeus (Carl von Linné) designed a floral clock using certain diurnal species of flowering plants. By arranging the selected species in a circular pattern, he designed a clock that indicated the time of day by observing which flowers were open and which ones were closed. For example, he discovered that the hawk's beard plant, opened its flowers at 6:30 am, whereas another species, the hawkbit, did not open its flowers until 7 am.
In 1924 Alexander Chizhevsky, graduate of Medical School at Moscow University, published interdisciplinary works: "Physical factors behind the process of history" and "Epidemiological catastrophes and periodic activity of the Sun" studying cycles in living organisms in connections with solar cycle and cycle of lunar phases. Chizhevsky developed a new discipline, Heliobiology, a branch of Astrobiology. In 1939 Chizhevsky was elected Honorary President of International Congress in Biological Physics, for his 1936 publication The Terrestrial Echo of Solar Storms, 366 pp. 1976, Moscow, (First published in 1936 in А.Л.Чижевский. Земное эхо солнечных бурь. full text in Russian ). However, soon Chizhevsky was arrested by the Soviet government and exiled to Siberia under the dictatorship of Joseph Stalin. Chizhevsky's publications were censored and his 1930s research of blood and electromagnetic parameters of erythrocytes in connection with cycles in human circadian system was banned; it was published in 1973, 40 years later. Chizhevsky's 1928 publication "Influence of Cosmos on human psychoses" was censored in the Soviet Union, albeit in 2003 this work was referenced in Journal of Circadian Rhythms article
The 1960 symposium at Cold Spring Harbor Laboratory seems to define the moment when researchers, as many women as men, from widely different fields discovered that they all were studying the same phenomenon. That well-attended meeting lay the groundwork for the field of chronobiology.
Franz Halberg of the University of Minnesota, who coined the word circadian, 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.
More recently, light therapy and melatonin administration have been explored by Dr. Alfred J. Lewy (OHSU) and other researchers as a means to reset animal and human circadian rhythms. Humans can be morning people or evening people; these variations are called chronotypes for which there are various assessment tools and biological markers.
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 (parametric, continuous, tonic, gradual vs. nonparametric, discrete, phasic, instantaneous, respectively; see this historical article, subscription required).
There is also a food-entrainable biological clock, which is not confined to the suprachiasmatic nucleus. It is located in the brain, possibly in the dorsomedial nucleus of the hypothalamus. Working with mice, Fuller et al concluded that the newly identified clock seems to be induced when food is restricted. It takes over control of such functions as activity timing, increasing the chances of the animal successfully locating food resources.
The unsubstantiated theory of biorhythms, which is said to describe a set of cyclic variations in human behaviour based on physiological and emotional cycles, is not a part of chronobiology.