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How do circadian rhythms work? On a molecular level, all biological clocks function as a collection of interacting feedback loops driven by a core set of 'clock' genes. Positive feedback loops activate the clock genes and negative feedback loops inhibit the clock genes. The architecture of biological clocks is similar for all organisms that exhibit circadian rhythms. The clocks are made up of three basic components: an input, a central oscillator, and an output. The input provides synchronization with the environment. The central oscillator is the core time keeping mechanism. The output transmits the timing information to control the organism's behaviors.

Image credit: www.ou.edu/wanglab/Circadian.html

Watch a video of how a negative feedback loop works in the fruitfly Drosophila.

What controls our circadian rhythm? Our circadian rhythms are controlled by both endogenous (internal) and environmental (external) cues. Endogenous control of circadian rhythms allows the oscillation pattern to continue in the absence of environmental cues. For instance, if a flower is kept in total darkness its petals will continue to open and close in a regular pattern. However, the phase of the pattern might shift because there are no external light-dark cues to synchronize the oscillation.

In humans, light is an external cue that activates receptors in our eyes. These receptors directly influence a structure within our brains called the suprachiasmatic nucleus or SCN. The SCN lies within the hypothalamus and is made up of a network of several thousand nerve cells. The firing of these nerve cells result in the cellular, physiological, and behavioral patterns observed.

Diagram of various neurological structures, including the suprachiasmatic nucleus (SCN), that are involved in human circadian rhythm control.
Image credit: http://thebrain.mcgill.ca

Watch a time-lapse video of a mouse's nocturnal behavior.