Circadian: Refers to events occurring within a 24-hour period, in the span of a full (24-hour) day, as in a circadian rhythm. Circadian rhythmicity is a fundamental property possessed by all organisms. These rhythms are driven by an internal time-keeping system: a clock. Changes in the external environment, particularly in the light-dark cycle, entrain this biologic clock. Under constant environmental conditions devoid of time cues, rhythms driven by the clock show a period near, but usually not exactly equal to, 24 hours.

Humans have an internal 24-hour clock which regulates our daily activities such as sleep and waking. Difficulties in readjusting our clock causes jet lag , work shift problems and some types of sleep disorders. Circadian clocks affect almost every level of our bodily functions.

A gene for the biological clock in a mouse was identified and cloned in 1997, the first such gene to be identified at the molecular level in a mammal. The identification of the "Clock" gene was proved by restoring a functioning biological clock to a line of mutant mice which had lost normal circadian rhythms. This was done by inserting DNA for the "Clock" gene into developing embryos that lacked it. They were born with a normal biological clock and passed the gene for it on to their descendants. The discovery of this gene regulating behavior was thus accompanied by simultaneous proof that the gene has been located by 'rescuing' the lost function of the gene.

The word "circadian" is a 20th-century invention. It was coined in 1959 from the Latin "circa" (around) + "diem" (a day). However, circadian rhythms have long been known. For example, the 18th-century Swedish botanist Carolus Linnaeus designed a garden that told time. He planted flowers in the garden that opened or closed their blossoms an hour apart around the clock.

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