Chronobiology is a field of science that examines periodic (cyclic) phenomena in living organisms. These cycles are known as biological rhythms. "Chrono" pertains to time and "biology" 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.
The variations of the duration of biological activity in living organisms occur for many essential biological processes. These occur in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.) and 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," literally meaning, "around a day"). Other important cycles include:
- Infradian rhythms, which are long-term cycles, such as the annual migration or reproduction cycles found in certain animals or the human menstrual cycle.
- Ultradian rhythms, which are short cycles, such as the 90-minute REM cycle, the 4-hour nasal cycle, or the 3-hour cycle of growth hormone production. They have periods of less than 24 hours.
- Tidal rhythms, commonly observed in marine life, which follow the (roughly) 12-hour transition from high to low tide and back.
Franz Halberg of the University of Minnesota is widely considered the "father of chronobiology."
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 . In 1751 Swedish botanist and naturalist Carolus Linnaeus 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. More recently, light therapy and melatonin administration have been explored by Dr. Alfred J. Lewy (OHSU) and other researchers as a means to reset abnormal animal and human circadian rhythms.
More recent research by William Rowe in the UK has discovered another colour/light clock called Chromo that helps the body understand what time it is based on a very simple colour and time standard. The theory allows you to control your circadian rhythm by using colour to release different hormones in your body, the two main ones being seratonin and melatonin.
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).
Related to, but not part of, chronobiology is the unsubstantiated theory of biorhythms. These are said to describe a set of cyclic variations in human behaviour. The basis of this theory are physiological and emotional cycles.
Chronobiology is a truly interdisciplinary interacting field of investigation. Jet lag is one of the medical conditions in which chronobiology research is deeply involved. It is also relevant to many other medical fields as the study of sleep disorders, endocrinology, geriatrics, sports medicine, space medicine, and more.
- Hastings, Michael, "The brain, circadian rhythms, and clock genes". Clinical review. BMJ 1998;317:1704-1707 19 December.
- U.S. Congress, Office of Technology Assessment, "Biological Rhythms: Implications for the Worker". U.S. Government Printing Office, September 1991. Washington, DC. OTA-BA-463. NTIS PB92-117589
- Ashikari, M., Higuchi, S., Ishikawa, F., and Tsunetsugu, Y., "Interdisciplinary Symposium on 'Human Beings and Environments': Approaches from Biological Anthropology, Social Anthropology and Developmental Psychology". Sunday, 25th August, 2002
- "Biorhythm experiment management plan", NASA, Ames Research Center. Moffett Field, 1983.
- "Biological Rhythms and Human Adaptation to the Environment". US Army Medical Research and Materiel Command (AMRMC), US Army Research Institute of Environmental Medicine.
- Ebert, D., K.P. Ebmeier, T. Rechlin, and W.P. Kaschka, "Biological Rhythms and Behavior", Advances in Biological Psychiatry. ISSN 0378-7354
- Halberg Chronobiology Laboratory at the University of Minnesota, founded by Franz Halberg, the "Father of Chronobiology"
- The University of Virginia offers an online tutorial on chronobiology.
- See the Science Museum of Virginia publication Can plants tell time? ar:ساعة بيولوجية