Some features of the human circadian (24-hour) biological clock History The earliest recorded account of a circadian process dates from the 4th century B.C.E., when Androsthenes, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree. 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. The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. The term circadian was coined by Franz Halberg in the 1950s. Criteria To be called circadian, a biological rhythm must meet these three general criteria: Origin The simplest known circadian clock is that of the prokaryotic cyanobacteria.
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