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Circadian rhythm

The meaning of «circadian rhythm»

A circadian rhythm (/sərˈkeɪdiən/), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours.[1] It can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (entrained by the environment). These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi and cyanobacteria.[2]

The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day". Processes with 24-hour cycles are more generally called diurnal rhythms; diurnal rhythms should not be called circadian rhythms unless they can be confirmed as endogenous, and not environmental.[3]

Although circadian rhythms are endogenous, they are adjusted to the local environment by external cues called zeitgebers (German for "time givers"), which include light, temperature and redox cycles. In clinical settings, an abnormal circadian rhythm in humans is known as a circadian rhythm sleep disorder.[4]

While there are multiple mentions of "natural body cycle" in Eastern and Native American cultures, the earliest recorded Western accounts of a circadian process date from the 4th century BC, when Androsthenes, a ship's 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]

In 1729, French scientist Jean-Jacques d'Ortous de Mairan conducted the first experiment designed to distinguish an endogenous clock from responses to daily stimuli. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica persisted, even when the plants were kept in constant darkness.[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. Auguste Forel, Ingeborg Beling, and Oskar Wahl conducted numerous experiments to determine whether this rhythm was attributable to an endogenous clock.[11] The existence of circadian rhythm was independently discovered in fruit flies in 1935 by two German zoologists, Hans Kalmus and Erwin Bünning.[12][13]

In 1954, an important experiment reported by Colin Pittendrigh demonstrated that eclosion (the process of pupa turning into adult) in Drosophila pseudoobscura was a circadian behaviour. He demonstrated that while temperature played a vital role in eclosion rhythm, the period of eclosion was delayed but not stopped when temperature was decreased.[14][13]

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