A biological rhythm that has a periodicity of about one day. This periodicity can be seen, for example, in the sleep cycle in animals and the growth cycles of plants.
A circadian rhythm is a roughly-24-hour cycle in the physiological processes of living beings, including plants, animals, fungi and cyanobacteria. The term "circadian", coined by Franz Halberg, comes from the Latin circa, "around", and dies, "day", meaning literally "about a day." The formal study of biological temporal rhythms such as daily, weekly, seasonal, and annual rhythms, is called chronobiology.
In a strict sense, circadian rhythms are endogenously generated, although they can be modulated by external cues such as sunlight and temperature. The first endogenous circadian oscillation was observed in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan who noticed that twenty four hour patterns in the movement of plant leaves continued even when the plants were isolated from external stimuli. Circadian rhythms may be defined by three criteria:
The rhythm persists in constant conditions (for example constant dark) with a period of about 24 hours The rhythm period can be reset by exposure to a light or dark pulse The rhythm is temperature compensated, meaning that it proceeds at the same rate within a range of temperatures.Origin
Circadian rhythms are believed to have originated in the earliest cells, with the purpose of protecting replicating DNA from high ultraviolet radiation during the daytime.
The simplest known circadian clock is that of the prokaryotic cyanobacteria. Recent research has demonstrated that the circadian clock of Synechococcus elongatus can be reconstituted in vitro with just the three proteins of their central oscillator. This clock has been shown to sustain a 22 hour rhythm over several days upon the addition of ATP. Indeed, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture: input - central oscillator - output, they do not share any homology.
Animal circadian rhythms
Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including human beings.
The rhythm is linked to the light-dark cycle. Animals kept in total darkness for extended periods eventually function with a "free-running" rhythm. The environmental cues that each day reset the rhythms are called Zeitgebers (German, literally "Time Givers"). Interestingly, totally blind subterranean mammals (e.g., blind mole rat Spalax sp.) are able to maintain their endogenous clock in absence of the external stimuli.
Free running organisms still have a consolidated sleep-wake cycle when in an environment shielded from external cues, but the rhythm is not entrained and may become out of phase with other circadian, or ultradian rhythms such as temperature and digestion.
The circadian "clock" in mammals is located in the suprachiasmatic nucleus (SCN), a distinct group of cells located in the hypothalamus. Destruction of the SCN results in the complete absence of a regular sleep/wake rhythm. It is interesting to note that, if cells from the SCN are removed and cultured, they maintain their own rhythm in the absence of external cues.
Recently, evidence has emerged that circadian rhythms are found in many cells in the body outside the SCN "master clock." Cells from many parts of the body appear to have "free-running" rhythms.
Disruption to rhythms usually has a negative effect in the short term. A number of other disorders, for example bipolar disorder and some sleep disorders are associated with irregular or pathological functioning of circadian rhythms. Recent research suggests that circadian rhythm disturbances found in bipolar disorder are positively influenced by lithium's effect on clock genes.
Disruption to rhythms in the longer term is believed to have significant adverse health consequences on peripheral organs outside the brain, particularly in the development or exacerbation of cardiovascular disease.
In addition, circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by drugs such as cocaine .
Circadian rhythms also play a part in the reticular activating system in the reticular formation.
Plant circadian rhythms
Plants are sessile organisms, and thus they are intimately associated with their environment. For example, the circadian clock makes an essential contribution to photosynthesis, with the outcome that the clock is believed to increase plant growth and survival. At the most fundamental level, circadian rhythms are the cyclical expression of genes in individual cells.
The study of circadian rhythms is therefore of particular interest for plant biologists.
Light and the biological clock
The ability of light to reset the biological clock depends on the phase response curve (to light). Depending on the phase of sleep, the light can advance or delay the circadian rhythm.
In addition to light intensity, wavelength (or color) of light is an important factor in the degree to which the clock is reset. Biological clock in total darkness: the Clock/MOP3 circadian system of the blind subterranean mole rat. Circadian genes in a blind subterranean mammal II: conservation and uniqueness of the three Period homologs in the blind subterranean mole rat, Spalax ehrenbergi superspecies. Sinauer, Sunderland Dvornyk V, Vinogradova ON, Nevo E (2003) Origin and evolution of circadian clock genes in prokaryotes. Day/night rhythms in gene expression of the normal murine heart. Science 217:1104–1111 Tomita J, Nakajima M, Kondo T, Iwasaki H (2005) No transcription–translation feedback in circadian rhythm of KaiC phosphorylation.
User Comments Add a comment…