A circadian rhythm is a roughly 24-hour cycle in the biochemical, physiological, or behavioural processes of living entities, including plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004,, animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also, fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell and cyanobacteria Cyanobacteria is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός (kyanós) = blue) (see bacterial circadian rhythms Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: in constant conditions (i.e. constant temperature and either constant light {LL} or constant darkness {DD}) they oscillate with a period that is close to, but not exactly, 24 hours in duration, (b)). The term "circadian" comes from the Latin Latin or sometimes Roman is an Italic language originally spoken in Latium and Ancient Rome. Although often considered a dead language, in view of the fact that it has no native, fluent speakers, Latin continues to be taught in schools and has been, and currently is, used in the process of new word production in modern languages from many circa Circa means "approximately", usually referring to a date, "around", and diem or dies, "day", meaning literally "approximately one day". The formal study of biological temporal rhythms such as daily, tidal Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. The tides occur with a period of approximately 12 hours and 25 minutes, and with an amplitude that is influenced by the alignment of the sun and moon and the shape of the near-shore, weekly, seasonal, and annual rhythms, is called chronobiology Chronobiology is a field of biology that examines periodic phenomena in living organisms and their adaptation to solar and lunar related rhythms. 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.

Although circadian rhythms are endogenous Endogenous substances are those that originate from within an organism, tissue, or cell . Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates. Their proviruses remain in the genome and are passed on to the next generation, they are adjusted (entrained In chronobiology, entrainment of a circadian system is the alignment of its own period and phase to the period and phase of an external rhythm. A common example is the entrainment of endogenous circadian rhythms to the daily light-dark cycle. Of the several possible cues, called zeitgeber (German for time-giver, synchronizer), which can contribute) to the environment by external cues called zeitgebers Zeitgeber is any exogenous (external) cue that synchronizes an organism's endogenous (internal) time-keeping system (clock) to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light. Other, non-photic, zeitgebers include temperature, social interactions, pharmacological manipulation and eating/drinking, the primary one of which is daylight Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime . This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space (that is, beyond the Earth's atmosphere) is.

Contents

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 Alexander III of Macedon , popularly known as Alexander the Great (Greek: Μέγας Ἀλέξανδρος, Mégas Aléxandros), was a Greeki[›] king (basileus) of Macedon. He is the most celebrated member of the Argead Dynasty and created one of the largest empires in ancient history. Born in Pella in 356 BC, Alexander received a classical, described diurnal Diurnality is an animal behavior characterized by activity during the day and sleeping at night. Animals that are not diurnal might be nocturnal or crepuscular (active primarily during twilight, i.e., at dusk and dawn). Many animal species are diurnal, including many mammals, insects, lizards and birds. The diurnal pattern is often controlled leaf movements of the tamarind Tamarind (from Latinization of Arabic: تمر هندي tamar eindi) is a tree in the family Fabaceae. The genus Tamarindus is monotypic (having only a single species) tree.[1] The first modern observation of endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan Jean-Jacques d'Ortous de Mairan was a French geophysicist in the 1700s; he noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica Mimosa pudica (pudica = shy), is a creeping annual or perennial herb often grown for its curiosity value: the compound leaves fold inward and droop when touched or shaken, re-opening minutes later. The species is native to South America and Central America, but is now a pantropical weed 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.[2] Joseph Takahashi discovered the genetic basis for the rodent circadian rhythm in 1994.[3][4]

The term "circadian" was coined by Franz Halberg in the late 1950s.[5]

Criteria

Historically, 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:

However, criterion 2 (that circadian rhythms persist equally precisely over a range of temperatures) is now understood to not be a factor. Thermal energy will affect the kinetics of all molecular processes, and likewise, temperature changes applied to the circadian clock mechanisms of many organisms (including fungi and even the SCN of mammals) has been shown to affect the frequency of the rhythm, as would be expected. In some instances, heat can provide a stronger zeitgeber than light.

Origin

This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be and removed. (October 2007)

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 Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet 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.[6] This is suggested by the maintenance (heritability) of circadian rhythms in fruit flies after several hundred generations in constant laboratory conditions,[7] as well as in creatures in constant darkness in the wild, and by the experimental elimination of behavioural but not physiological circadian rhythms in quail.[8][9]

The simplest known circadian clock is that of the prokaryotic cyanobacteria Cyanobacteria is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός (kyanós) = blue). 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 Adenosine-5'-triphosphate is a multifunctional nucleotide used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism. It is produced by photophosphorylation and cellular respiration and used by enzymes and structural. Previous explanations of the prokaryotic The prokaryotes are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles. The word prokaryote comes from the Greek πρό- (pro-) & circadian timekeeper were dependent upon a DNA transcription/translation feedback mechanism.

In 1971, Ronald J. Konopka and Seymour Benzer Seymour Benzer was an accomplished American physicist, molecular biologist and behavioral geneticist. With a career that started with the molecular biology revolution of the 1950s, Seymour Benzer was to the end very active as a researcher, where he led a productive lab as the James G. Boswell Professor of Neuroscience, Emeritus, at the California first identified a genetic component of the biological clock using the fruit fly as a model system. Three mutant lines of flies displayed aberrant behaviour: 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".[10] 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. Many more genetic components of the biological clock are now known. 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.[11] At the same time, different cells may communicate with each other resulting in a synchronised 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 synchronise the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the eyes Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons. The simplest photoreceptors connect light to movement . In higher organisms complex neural pathways exist that connect the eye, via the optic nerve to the visual cortex and other areas of the brain. Complex optical systems with resolving power have come in travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronised. 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 Sleep is a naturally recurring state of relatively suspended sensory and motor activity, characterized by total or partial unconsciousness and the inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. Sleep is a and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave Electroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain. In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from multiple electrodes placed on the scalp. In activity, hormone A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; production, cell regeneration and other biological activities. In addition, photoperiodism Many flowering plants use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. In a further subdivision, obligate photoperiodic plants absolutely require a long or short enough night before flowering, whereas facultative photoperiodic plants, 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.[12] ”

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 Free-running sleep experiments can involve any organism which sleeps. Freerunning sleep is sleep which is not adjusted, entrained, to the 24-hour cycle in nature nor to any artificial cycle. Such experiments are used in the study of circadian and other rhythms in biology. Subjects are shielded from all time cues, often by a constant light protocol, rhythm. Each "day", their sleep cycle is pushed back or forward, depending on whether their endogenous Endogenous substances are those that originate from within an organism, tissue, or cell . Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates. Their proviruses remain in the genome and are passed on to the next generation period is shorter or longer than 24 hours. The environmental cues that each day reset the rhythms are called zeitgebers Zeitgeber is any exogenous (external) cue that synchronizes an organism's endogenous (internal) time-keeping system (clock) to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light. Other, non-photic, zeitgebers include temperature, social interactions, pharmacological manipulation and eating/drinking (from the German, Time Givers).[13] It is interesting to note that totally-blind subterranean mammals (e.g., blind mole rat Blind mole rats are one of many types of rodents that are referred to as mole rats. The hystricognath mole rats of the family Bathyergidae are completely unrelated, whereas some other forms are also in the family Spalacidae. Zokors and root and bamboo rats (subfamily Rhizomyinae) are spalacids sometimes referred to as mole rats. Blind mole rats 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.[14]

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 Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable and digestion In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the esophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food , and chemical alteration of some. The.[citation needed]

Recent research has influenced the design of spacecraft Human spaceflight is spaceflight with a human crew and possibly passengers. This makes it unlike robotic space probes or remotely-controlled satellites. Human spaceflight is sometimes called manned spaceflight, a term now deprecated by major space agencies in favor of its gender-neutral alternative environments, as systems that mimic the light/dark cycle have been found to be highly beneficial to astronauts.[citation needed]

Arctic animals

Norwegian researchers at the University of Tromsø have shown that some Arctic animals (ptarmigan The Rock Ptarmigan is a medium-sized (31–35 cm or 12–14 in.) gamebird in the grouse family. It is known simply as Ptarmigan in Europe and colloquially as Snow Chicken or Partridge in North America, where it is the official bird for the territory of Nunavut, Canada, and the official game bird for the province of Newfoundland and Labrador,, reindeer The reindeer , also known as the caribou in North America, is a deer from the Arctic and Subarctic, including both resident and migratory populations. While overall widespread and numerous, some of its subspecies are rare and one (or two, depending on taxonomy) have already gone extinct) 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 The 70th parallel north is a circle of latitude that is 70 degrees north of the Earth's equatorial plane 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 summer and the constant dark of winter.[15][16]

However, another study in northern Alaska found that ground squirrels The ground squirrels are the members of the Sciuridae most closely related to the genus Marmota. They make up the tribe Marmotini in the large and mainly terrestrial squirrel subfamily Xerinae, and containing six living genera. Well-known members of this largely Holarctic group are the marmots , the susliks (Spermophilus), and the prairie dogs ( and porcupines Porcupines are rodents with a coat of sharp spines, or quills, that defend them from predators. They are indigenous in both the Old World and the New World. Porcupines are the third largest of the rodents, behind the capybara and the beaver. Most porcupines are about 25–36 in long, with an 8–10 in (20–25 cm) long tail. Weighing between 12–3 strictly maintained their circadian rhythms through 82 days and nights of sunshine. The researchers speculate that these two small mammals see that the apparent distance between the sun and the horizon is shortest once a day, and, thus, a sufficient signal to adjust by.[17]

Butterfly migration

The navigation of the fall migration of the Eastern North American monarch butterfly The Monarch is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all North American butterflies. Since the 19th century, it has been found in New Zealand, and in Australia since 1871 where it is called the Wanderer. In Europe it is resident in the Canary Islands, the Azores, and Madeira, and is (Danaus plexippus) to their overwintering grounds in central Mexico uses a time-compensated sun compass that depends upon a circadian clock in their antennae.[18][19]

Biological clock in mammals

Diagram illustrating the influence of light and darkness on circadian rhythms and related physiology Physiology is the science of the functioning of living systems. It is a subcategory of biology. In physiology, the scientific method is applied to determine how organisms, organ systems, organs, cells and biomolecules carry out the chemical or physical function that they have in a living system. The word physiology is from Ancient Greek: φύσις and behaviour through the suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regulate many different body functions in a 24-hour cycle, using around 20,000 neurons in humans.

The primary circadian "clock" in mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not is located in the suprachiasmatic nucleus The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regulate many different body functions in a 24-hour cycle, using around 20,000 neurons (or nuclei) (SCN), a pair of distinct groups of cells The cell is the functional basic unit of life. It was discovered by Robert Hooke and is the functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, located in the hypothalamus The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland. Destruction of the SCN results in the complete absence of a regular sleep–wake rhythm. The SCN receives information about illumination through the eyes. The retina The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These of the eye contains not only the "classical" photoreceptors which are used for vision but ganglion cells which respond to light and are called photosensitive ganglion cells.

These cells contain the photo pigment melanopsin and their signals follow a pathway called the retinohypothalamic tract, leading to the SCN. If cells from the SCN are removed and cultured, they maintain their own rhythm in the absence of external cues.

The SCN takes the information on the lengths of the day and night from the retina, interprets it, and passes it on to the pineal gland, a tiny structure shaped like a pine cone and located on the epithalamus. In response, the pineal secretes the hormone melatonin. Secretion of melatonin peaks at night and ebbs during the day and its presence provides information about night-length.

The circadian rhythms of humans can be entrained to slightly shorter and longer periods than the Earth's 24 hours. Researchers at Harvard have recently shown that human subjects can at least be entrained to a 23.5-hour cycle and a 24.65-hour cycle (the latter being the natural solar day-night cycle on the planet Mars).[20]

Determining the human circadian rhythm

The classic phase markers for measuring the timing of a mammal's circadian rhythm are:[21]

For temperature studies, people must remain awake but calm and semi-reclined in near darkness while their rectal temperatures are taken continuously. The average human adult's temperature reaches its minimum at about 05:00 (5 a.m.), about two hours before habitual wake time, though variation is great among normal chronotypes.

Melatonin is absent from the system or undetectably low during daytime. Its onset in dim light, dim-light melatonin onset (DLMO), at about 21:00 (9 p.m.) can be measured in the blood or the saliva. Its major metabolite can also be measured in morning urine. Both DLMO and the midpoint (in time) of the presence of the hormone in the blood or saliva have been used as circadian markers.

However, newer research indicates that the melatonin offset may be the most reliable marker. Benloucif et al. in Chicago in 2005 found that melatonin phase markers were more stable and more highly correlated with the timing of sleep than the core temperature minimum. They found that both sleep offset and melatonin offset were more strongly correlated with the various phase markers than sleep onset. In addition, the declining phase of the melatonin levels was more reliable and stable than the termination of melatonin synthesis.[21]

One method used for measuring melatonin offset is to analyse a sequence of urine samples throughout the morning for the presence of the melatonin metabolite 6-sulphatoxymelatonin (aMT6s). Laberge et al. in Quebec in 1997 used this method in a study that confirmed the frequently found delayed circadian phase in healthy adolescents.[22]

Outside the "master clock"

More-or-less independent circadian rhythms are found in many organs and cells in the body outside the suprachiasmatic nuclei (SCN), the "master clock". These clocks, called peripheral oscillators, are found in the oesophagus, lungs, liver, pancreas, spleen, thymus, and the skin.[23] Though oscillators in the skin respond to light, a systemic influence has not been proven so far.[24][25] There is also some evidence that the olfactory bulb and prostate may experience oscillations when cultured, suggesting that these structures may also be weak oscillators.

Furthermore, liver cells, for example, appear to respond to feeding rather than to light. Cells from many parts of the body appear to have freerunning rhythms.

Light and the biological clock

Light resets the biological clock in accordance with the phase response curve (PRC). Depending on the timing, light can advance or delay the circadian rhythm. Both the PRC and the required illuminance vary from species to species and lower light levels are required to reset the clocks in nocturnal rodents than in humans.

Lighting levels that affect the circadian rhythm in humans are higher than the levels usually used in artificial lighting in homes. According to some researchers[26] the illumination intensity that excites the circadian system has to reach up to 1000 lux striking the retina. In addition to light intensity, wavelength (or colour) of light is a factor in the entrainment of the body clock. Melanopsin is most efficiently excited by blue light, 420–440 nm[27] according to some researchers while others have reported 470–485 nm.

It is thought that the direction of the light may have an effect on entraining the circadian rhythm;[26] light coming from above, resembling an image of a bright sky, has greater effect than light entering our eyes from below.

According to a 2010 study completed by the Lighting Research Center, daylight has a direct effect on circadian rhythms and, consequently, on performance and well-being. The research showed that students who experience disruption in lighting schemes in the morning consequently experience disruption in sleeping patterns. The change in sleeping patterns may lead to negatively impacted student performance and alertness. Removing circadian light in the morning delays the dim light melatonin onset by 6 minutes a day, for a total of 30 minutes for five days.[28]

Enforced longer cycles

Modern research under very controlled conditions has shown the human period for adults to be just slightly longer than 24 hours on average. Czeisler et al. at Harvard found the range for normal, healthy adults of all ages to be quite narrow: 24 hours and 11 minutes ± 16 minutes. The "clock" resets itself daily to the 24-hour cycle of the Earth's rotation.[29]

The 28-hour day is presented as a concept of time management.[30] It builds on the fact that the week of seven days at 24 hours and a "week" of six days at 28 hours both equal a week of 168 hours. To live on the 28-hour day and six-day week would require staying awake for 19 to 20 hours and sleeping for eight to nine hours. Each "day" on this system has a unique light/dark pattern.

Studies by Nathaniel Kleitman[31] in 1938 and by Derk-Jan Dijk and Charles Czeisler[32][33] in 1994/5 have put human subjects on enforced 28-hour sleep–wake cycles, in constant dim light and with other time cues suppressed, for over a month. Because normal people cannot entrain to a 28-hour day,[34] this is referred to as a forced desynchrony protocol. Sleep and wake episodes are uncoupled from the endogenous circadian period of about 24.18 hours and researchers are allowed to assess the effects of circadian phase on aspects of sleep and wakefulness including sleep latency and other functions.[35]

Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping. Early investigators determined the human circadian period to be 25 hours or more. They went to great lengths to shield subjects from time cues and daylight, but they were not aware of the effects of indoor electric lights. The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase. These results became well known.[29] Researchers allowed subjects to keep electric lighting on in the evening, as it was thought at that time that a couple of 60W bulbs would not have a resetting effect on the circadian rhythms of humans. More recent research[citation needed] has shown that adults have a built-in day, which averages just over 24 hours, that indoor lighting does affect circadian rhythms and that most people attain their best-quality sleep during their chronotype-determined sleep periods.

Human health

Timing of medical treatment in coordination with the body clock may significantly increase efficacy and reduce drug toxicity or adverse reactions. For example, appropriately timed treatment with angiotensin converting enzyme inhibitors (ACEi) may reduce nocturnal blood pressure and also benefit left ventricular (reverse) remodelling.[citation needed]

A short nap during the day does not affect circadian rhythms.

A number of studies have concluded that a short period of sleep during the day, a power-nap, does not have any effect on normal circadian rhythm, but can decrease stress and improve productivity.[36][37]

There are many health problems associated with disturbances of the human circadian rhythm, such as seasonal affective disorder (SAD), delayed sleep phase syndrome (DSPS) and other circadian rhythm disorders.[38] Circadian rhythms also play a part in the reticular activating system, which is crucial for maintaining a state of consciousness. In addition, a reversal in the sleep–wake cycle may be a sign or complication of uremia,[39] azotemia or acute renal failure.

Studies have also shown that light has a direct effect on human health because of the way it influences the circadian rhythms.[40][41][42]

Disruption

Disruption to rhythms usually has a negative effect. Many travellers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation and insomnia.

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.[43]

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.[44] The suppression of melatonin production associated with the disruption of the circadian rhythm may increase the risk of developing cancer.[45]

Effect of drugs

Circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by drugs such as cocaine.[46][47] Moreover, genetic manipulations of clock genes profoundly affect cocaine's actions.[48]

See also

References

Bibliography

Notes

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'Circadian Rhythm' Blogs
How Cancer Cells Loose Their ( Circadian ) Rhythm
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Immortality and uncontrolled cell division are the fundamental differences between cancer cells and normal cells. Link: How Cancer Cells Loose Their (. Circadian. ) . Rhythm. .

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'Circadian Rhythm' Answers
Is my Circadian Rhythm off?
Q. I can't seem to get back to normal. I am drousy all day long, but as soon as night hits, my mind starts racing and i don't want to go to bed. I've always been a nite person but i can't fall asleep at a reasonable time lately.
Asked by tigerkitty2002 - Fri Nov 10 16:44:52 2006 - - 1 Answers - 0 Comments

A. keep at it, you can use bnadryl or tylenol pm or melatonin as sleeping aids to help you sleep at night, avoid caffeine, no tv or computer about an hour or two before bed time, slow and steady, you will change it all, good luck
Answered by HK3738 - Tue Nov 14 00:21:10 2006

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