Circadian Clock

A circadian clock, or circadian oscillator, is a biochemical mechanism that oscillates with a period of 24 hours and is coordinated with the day-night cycle. Circadian clocks are the central mechanisms which drive circadian rhythms. They consist of three major components:

1. A central oscillator with a period of about 24 hours that keeps time
2. A series of input pathways to this central oscillator to allow entrainment of the clock
3. A series of output pathways tied to distinct phases of the oscillator that regulate overt rhythms in biochemistry, physiology, and behavior throughout the organism

The clock is reset as the environment changes through an organism's ability to sense external 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 basic molecular mechanisms of the biological clock have been defined in vertebrate species, Drosophila melanogaster, plants, fungi, and bacteria.

Recent studies employing genomic approaches have further elucidated understanding of the circadian oscillator mechanism by providing a large-scale view into an organism's network and genetic architecture.