A circadian rhythm is a roughly 24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria (see bacterial circadian rhythms). The term "circadian", coined by Franz Halberg, comes from the Latin circa , "around," and diem or dies , "day", meaning literally "approximately one day." The formal study of biological temporal rhythms such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology.
Circadian rhythms are endogenously generated, and can be entrained by external cues, called zeitgebers, the primary one of which is daylight.
History
The earliest known account of a circadian rhythm dates from the 4th century BC, when Androsthenes, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree. The first modern observation of endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in the 1700s; he noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were isolated from external stimuli.
In 1918, J. S. Szymanski showed that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature. Joseph Takahashi discovered the genetic basis for the rodent circadian rhythm in 1994.
Criteria
To differentiate genuinely endogenous circadian rhythms from coincidental or apparent ones, three general criteria must be met: 1) the rhythms persist in the absence of cues, 2) they persist equally precisely over a range of temperatures, and 3) the rhythms can be adjusted to match the local time:
- The rhythm persists in constant conditions (for example, constant dark) with a period of about 24 hours. The rationale for this criterion is to distinguish circadian rhythms from those "apparent" rhythms that are merely responses to external periodic cues. A rhythm cannot be declared to be endogenous unless it has been tested in conditions without external periodic input.
- The rhythm is temperature-compensated, i.e., it maintains the same period over a range of temperatures. The rationale for this criterion is to distinguish circadian rhythms from other biological rhythms arising due to the circular nature of a reaction pathway. At a low enough or high enough temperature, the period of a circular reaction may reach 24 hours, but it will be merely coincidental.
- The rhythm can be reset by exposure to an external stimulus. The rationale for this criterion is to distinguish circadian rhythms from other imaginable endogenous 24-hour rhythms that are immune to resetting by external cues and, hence, do not serve the purpose of estimating the local time. Travel across time zones illustrates the necessity of the ability to adjust the biological clock so that it can reflect the local time and anticipate what will happen next. Until rhythms are reset, a person usually experiences jet lag.
Origin
Photosensitive proteins and circadian rhythms are believed to have originated in the earliest cells, with the purpose of protecting the replicating of DNA from high ultraviolet radiation during the daytime. As a result, replication was relegated to the dark. The fungus Neurospora , which exists today, retains this clock-regulated mechanism.
Circadian rhythms allow organisms to anticipate and prepare for precise and regular environmental changes; they have great value in relation to the outside world. The rhythmicity appears to be as important in regulating and coordinating internal metabolic processes, as in coordinating with the environment. This is suggested by the maintenance (heritability) of circadian rhythms in fruit flies after several hundred generations in constant laboratory conditions (Sheeba et al. 1999) as well as in creatures in constant darkness in the wild, and by the experimental elimination of behavioral but not physiological circadian rhythms in quail (Guyomarc'h et al. 1998, Zivkovic et al. 1999).
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. Previous explanations of the prokaryotic circadian timekeeper were dependent upon a DNA transcription / translation feedback mechanism.
It is an unanswered question whether circadian clocks in eukaryotic organisms require translation/transcription-derived oscillations, for, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture, input - central oscillator - output, they do not share any homology. This implies probable independent origins.
In 1971, Ronald J. Konopka and Seymour Benzer first identified a genetic component of the biological clock using the fruit fly as a model system. Three mutant lines of flies displayed aberrant behavior - one had a shorter period, another had a longer one, and the third had none. All three mutations mapped to the same gene, which was named period . The same gene was identified to be defective in the sleep disorder FASPS (Familial advanced sleep phase syndrome) in human beings thirty years later - underscoring the conserved nature of the molecular circadian clock through evolution. We now know many more genetic components of the biological clock. Their interactions result in an interlocked feedback loop of gene products resulting in periodic fluctuations that the cells of the body interpret as a specific time of the day.
A great deal of research on biological clocks was done in the latter half of the 20th century. It is now known that the molecular circadian clock can function within a single cell; i.e., it is cell-autonomous. At the same time, different cells may communicate with each other resulting in a synchronized output of electrical signaling. These may interface with endocrine glands of the brain to result in periodic release of hormones. The receptors for these hormones may be located far across the body and synchronize the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the eyes travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronized. This is how the timing of, for example, sleep/wake, body temperature, thirst, and appetite are coordinately controlled by the biological clock.
Importance in animals
Circadian rhythmicity is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave activity, hormone production, cell regeneration and other biological activities. In addition, photoperiodism, the physiological reaction of organisms to the length of day or night, is vital to both plants and animals, and the circadian system plays a role in the measurement and interpretation of day length.
Timely prediction of seasonal periods of weather conditions, food availability or predator activity is crucial for survival of many species. Although not the only parameter, the changing length of the photoperiod ('daylength') is the most predictive environmental cue for the seasonal timing of physiology and behavior, most notably for timing of migration, hibernation and reproduction.
Impact of light-dark cycle
The rhythm is linked to the light-dark cycle. Animals, including humans, kept in total darkness for extended periods eventually function with a freerunning rhythm. Each "day," their sleep cycle is pushed back or forward, depending on whether their endogenous period is shorter or longer than 24 hours. The environmental cues that each day reset the rhythms are called Zeitgebers (from the German, Time Givers ). It is interesting to note that totally-blind subterranean mammals (e.g., blind mole rat Spalax sp.) are able to maintain their endogenous clocks in the apparent absence of external stimuli. Although they lack image-forming eyes, their photoreceptors (detect light) are still functional; as well, they do surface periodically.
Freerunning organisms that normally have one consolidated sleep episode will still have it when in an environment shielded from external cues, but the rhythm is, of course, not entrained to the 24-hour light/dark cycle in nature. The sleep/wake rhythm may, in these circumstances, become out of phase with other circadian or ultradian rhythms such as temperature and digestion.
Recent research has influenced the design of spacecraft environments, as systems that mimic the light/dark cycle have been found to be highly beneficial to astronauts.
Arctic animals
Norwegian researchers at the University of Tromsø have shown that some Arctic animals (ptarmigan, reindeer) show circadian rhythms only in the parts of the year that have daily sunrises and sunsets. In one study of reindeer, animals at 70 degrees North showed circadian rhythms in the autumn, winter, and spring, but not in the summer. Reindeer at 78 degrees North showed such rhythms only autumn and spring. The researchers suspect that other Arctic animals as well may not show circadian rhythms in the constant light of su
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