Animal biological rhythms

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.

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, 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.

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References & Bibliography


Books

 * Hastings, Michael, "The brain, circadian rhythms, and clock genes". Clinical review. BMJ 1998;317:1704-1707 19 December.

Papers

 * Google Scholar