Circadian rhythm

Circadian rhythm
Biological clock human.svg
Some features of the human circadian (24-hour) biological clock
Frequency24 hr

A circadian rhythm (n/) is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.[1]

The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day". The formal study of biological temporal rhythms, such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology. Processes with 24-hour oscillations are more generally called diurnal rhythms; strictly speaking, they should not be called circadian rhythms unless their endogenous nature is confirmed.[2]

Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers (from German, "time giver"), which include light, temperature and redox cycles. In medical science, an abnormal circadian rhythm in humans is known as circadian rhythm disorder.[3]

In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young "for their discoveries of molecular mechanisms controlling the circadian rhythm" in fruit flies.[4]

History

The earliest recorded account of a circadian process dates from the 4th century BC, when Androsthenes, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree.[5] The observation of a circadian or diurnal process in humans is mentioned in Chinese medical texts dated to around the 13th century, including the Noon and Midnight Manual and the Mnemonic Rhyme to Aid in the Selection of Acu-points According to the Diurnal Cycle, the Day of the Month and the Season of the Year.[6]

The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were kept in constant darkness, in the first experiment to attempt to distinguish an endogenous clock from responses to daily stimuli.[7][8]

In 1896, Patrick and Gilbert observed that during a prolonged period of sleep deprivation, sleepiness increases and decreases with a period of approximately 24 hours.[9] 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.[10] In the early 20th century, circadian rhythms were noticed in the rhythmic feeding times of bees. Extensive experiments were done by Auguste Forel, Ingeborg Beling, and Oskar Wahl to see whether this rhythm was due to an endogenous clock.[11] The existence of circadian rhythm was independently discovered in the fruit fly Drosophila melanogaster in 1935 by two German zoologists, Hans Kalmus and Erwin Bünning.[12][13]

In 1954, an important experiment was reported by Colin Pittendrigh who showed that eclosion (the process of pupa turning into adult) in D. pseudoobscura was a circadian behaviour. He demonstrated that temperature played a vital role in eclosion rhythm, the period of eclosion was delayed but not stopped when temperature was decreased.[14] It was an indication that circadian rhythm was controlled by an internal biological clock.[13] The term circadian was coined by Franz Halberg in 1959.[15] According to Halberg's original definition:

The term "circadian" was derived from circa (about) and dies (day); it may serve to imply that certain physiologic periods are close to 24 hours, if not exactly that length. Herein, "circadian" might be applied to all "24-hour" rhythms, whether or not their periods, individually or on the average, are different from 24 hours, longer or shorter, by a few minutes or hours.[16][17]

In 1977, the International Committee on Nomenclature of the International Society for Chronobiology formally adopted the definition, which states:

Circadian: relating to biologic variations or rhythms with a frequency of 1 cycle in 24 ± 4 h; circa (about, approximately) and dies (day or 24 h). Note: term describes rhythms with an about 24-h cycle length, whether they are frequency-synchronized with (acceptable) or are desynchronized or free-running from the local environmental time scale, with periods of slightly yet consistently different from 24-h.[18]

Ron Konopka and Seymour Benzer identified the first clock mutant in Drosophila in 1971 and called it "period" (per) gene, the first discovered genetic determinant of behavioral rhythmicity.[19] per gene was isolated in 1984 by two teams of researchers. Konopka, Jeffrey Hall, Michael Roshbash and their team showed that per locus is the centre of the circadian rhythm, and that loss of per stops circadian activity.[20][21] At the same time, Michael W. Young's team reported similar effects of per, and that the gene covers 7.1-kilobase (kb) interval on the X chromosome and encodes a 4.5-kb poly(A)+ RNA.[22][23] They went on to discover the key genes and neurones in Drosophila circadian system, for which Hall, Rosbash and Young received the Nobel Prize in Physiology or Medicine 2017.[4]

Joseph Takahashi discovered the first mammalian circadian clock mutation (clockΔ19) using mice in 1994.[24][25] However, recent studies show that deletion of clock does not lead to a behavioral phenotype (the animals still have normal circadian rhythms), which questions its importance in rhythm generation.[26][27]

Other Languages
العربية: نظم يوماوي
български: Циркаден ритъм
dansk: Døgnrytme
Esperanto: Cirkadia ritmo
français: Rythme circadien
한국어: 활동일 주기
Bahasa Indonesia: Ritme sirkadian
Kreyòl ayisyen: Ritm sikadyen
Bahasa Melayu: Ritma sirkadian
Nederlands: Circadiaan ritme
日本語: 概日リズム
norsk: Døgnrytme
norsk nynorsk: Døgnrytme
polski: Rytm dobowy
português: Ritmo circadiano
română: Ritm circadian
Simple English: Circadian rhythm
slovenščina: Cirkadiani ritem
srpskohrvatski / српскохрватски: Cirkadijani ritam
svenska: Dygnsrytm
Türkçe: Biyolojik saat
українська: Циркадний ритм
中文: 昼夜节律