A circadian rhythm is a roughly 24-hour cycle in the physiological and behavioral processes of plants, animals, fungi, and cyanobacteria. (The term "circadian" comes from the Latin circa, "around," and dies, "day," meaning literally "around a day.") French scientist Jean-Jacques d'Ortous de Mairan discovered circadian rhythms in 1729 when he observed mimosa plants kept in constant darkness continued to unfold and fold their leaves each day. The formal study of biological temporal rhythms, including daily, weekly, seasonal, and annual patterns, is called chronobiology.
Circadian rhythms represent a type of entrainment, an innate physiological response by which organisms enter into harmony with the environment. All living beings, from the simplest to the most complex, entrain with the environment and other living organisms (Burns 2006). It is a basic biological characteristic of life.
Normally, daily environmental factors such as sunlight and temperature provide timing cues for the synchronization of the rhythm (Takahashi 1982). Early researchers observed that some sort of "internal" rhythm must exist because plants and animals did not react immediately to artificially induced changes in daily rhythms. In the absence of daily environmental cues, plants and animals eventually adjust their internal clock to a new pattern, as long as the period is sufficiently regular and not too far off the norm for the species. Overall, circadian rhythms are defined by three criteria:
Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. There are clear patterns of brain wave activity, hormone production, cell regeneration, and other biological activities linked to this daily cycle.
The rhythm is linked to the light-dark cycle. Animals kept in total darkness for extended periods eventually function with a "free-running" rhythm, meaning that their sleep-wake cycle persists even though environmental cues are absent. Each "day," their sleep cycle is pushed back or forward—depending whether they are nocturnal (sleeps during day and is active at night) or diurnal (active during the day) animals—by approximately one hour. Free-running rhythms of diurnal animals are close to 25 hours. The human free-running circadian rhythm is just over 24 hours, not 25 hours, as many textbooks assert (Czeisler 1999). The environmental cues that reset the rhythms each day are called Zeitgebers.
Though free-running organisms still have a consolidated sleep-wake cycle when in an environment shielded from external cues, the rhythm is not entrained. (Entrainment can be defined as the process whereby connected oscillating systems with similar periods fall into synchrony, such as when the menstrual cycles of women living together synchronize or the actual sleep schedule matches the circadian rhythm). They may become out of phase with other circadian or ultradian (regular recurrence in less than 24 hours) rhythms such as temperature and digestion. Research in this area has influenced the design of spacecraft environments, as systems that mimic the light/dark cycle have been found to be highly beneficial to astronauts.
The circadian "master 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. Contributing to this clock are photoreceptors found in the retina that are known as melanopsin ganglia. These light-detecting cells, which contain a photo pigment called melanopsin, do not send information to the visual parts of the brain; instead, they follow the retinohypothalamic tract, a pathway leading to the SCN. Researchers have found that if cells from the SCN are removed and cultured, they maintain their own rhythm in the absence of external cues.
The SCN is believed to take the information on day length from the retina, interpret it, and pass it on to the pineal gland (a pea-like structure found on the epithalamus), which then secretes the hormone melatonin in response. Secretion of melatonin peaks at night and ebbs during the day. The SCN does not appear to be able to react rapidly to changes in the light/dark cues.
In the early twenty-first century, evidence emerged that circadian rhythms are found in many cells in the body, outside the SCN master clock. For example, liver cells appear to respond to feeding rather than light. 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. Many travelers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation, and insomnia. A number of other disorders, such as bipolar disorder and sleep disorder, are associated with irregular or pathological functioning of the circadian rhythms.
Researchers suggest in 2006 that circadian rhythm disturbances found in bipolar disorders are positively influenced by lithium, through its blocking of an enzyme and stabilizing the body clock (Yin 2006).
In addition, circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by abuse of drugs such as cocaine (Uz 2003; Kurtuncu 2004). Moreover, genetic manipulations of clock genes profoundly affect cocaine's actions (McClung 2005).
Circadian rhythms also play a part in the reticular activating system in reticular formation.
The ability to synchronize with daily changes in temperature and light is of great advantage to plants, which, as sessile organisms (which do not move about) 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. As days grow shorter and cooler, plants are able to change the expression of their genes to prepare for the end of the growing season and for winter. At the most fundamental level, circadian rhythms are the cyclical expression of genes in individual cells. This cyclical expression is controlled by a central clock, which responds to light and temperature inputs.
The study of circadian rhythms is therefore of particular interest to plant biologists. Many of the circadian-controlled genes are involved in chilling and freezing tolerance and photosynthesis. A better understanding of these genes could allow the creation of stress-tolerant plants that are better able to survive in cold temperatures and grow with increased vigor. This development would allow the expansion of both growing seasons and the growth range for many economically important crops.
In addition to light intensity, wavelength (or color) of light is an important factor in the degree to which the clock is reset. Melanopsin is most efficiently excited by blue light (420-440 nm) (Newman 2003).
Circadian rhythms are believed to have originated in the earliest cells to provide protection for replicating DNA from high ultraviolet radiation during day-time. As a result, replication was relegated to the dark. The fungus Neurospora, which exists today, retains this clock-regulated mechanism. Remarkably, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture (input - central oscillator - output), they do not share any homology. This distinction may imply their probable independent origin (Ditty 2003; Dvornyk 2003).
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