Chronobiology

Chronobiology is a field of science that examines periodic (cyclic) phenomena in living organisms. "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.

Description
The variations of the 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 literally, "around a day."). This and other many other important cycles are also studied, including:


 * 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 in sleep 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.

History
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, Carolus Linnaeus, Swedish botanist/naturalist designed  a flower garden clock, botanical clock, or petal time 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 circadian rhythms.

Other fields
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. Some people consider it pseudoscience and others protoscience.

Research publications

 * 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

External articles

 * The University of Virginia offers an online tutorial on chronobiology.
 * See the Science Museum of Virginia publication Can plants tell time?

Chronobiologie Chronobiologie