<P> 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 . </P> <P> 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 . </P> <P> 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 . 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 . 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 . Ron Konopka and Seymour Benzer isolated the first clock mutant in Drosophila in the early 1970s and mapped the "period" gene, the first discovered genetic determinant of behavioral rhythmicity . Joseph Takahashi discovered the first mammalian circadian clock mutation (clockΔ19) using mice in 1994 . 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 . </P> <P> The term circadian was coined by Franz Halberg in the 1950s . </P>

The first circadian gene to be discovered in mammals was
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