Circadian Clock - The body's time keeper

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 

To be called circadian, a biological rhythm must meet these four general criteria:

1. The rhythms repeat once a day (they have a 24-hour period). 
2. The rhythms persist in the absence of external cues (endogenous). 
3. The rhythms can be adjusted to match the local time (entrainable). 
4. The rhythms maintain circadian periodicity over a range of physiological temperatures; they exhibit temperature compensation. 

Tomorrow we will talk about Circadian rhythm and JET LAG