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 . 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. These rhythms allow organisms to anticipate and prepare for precise and regular environmental changes.
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 mammalian circadian rhythm in 1994.Wikipedia
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