Circadian clock

The circadian clock, or circadian oscillator, in most living things makes it possible for organisms to coordinate their biology and behavior with daily environmental changes in the day-night cycle. The term circadian derives from the Latin circa (about) diem (a day), since when taken away from external cues (such as the day-night cycle), they do not run to exactly 24 hours. Clocks in humans in a lab in constant low light, for example, will average about 24.2 hours per day, rather than 24 hours exactly. Hence the term circadian.The normal body clock oscillates with a period of exactly 24 hours when it receives daily corrective signals from the environment, primarily daylight and darkness. Circadian clocks are the central mechanisms that drive circadian rhythms. They consist of three major components: A central biochemical oscillator with a period of about 24 hours that keeps time A series of input pathways to this central oscillator to allow entrainment of the clock A series of output pathways tied to distinct phases of the oscillator that regulate overt rhythms in biochemistry, physiology, and behavior throughout an organism.The clock is reset as an organism senses environmental time cues of which the primary one is light. Circadian oscillators are ubiquitous in tissues of the body where they are synchronized by both endogenous and external signals to regulate transcriptional activity throughout the day in a tissue-specific manner. The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging. The basic molecular mechanisms of the biological clock have been defined in vertebrate species, Drosophila melanogaster, plants, fungi, bacteria, and presumably also in Archaea.