The characteristics of this rhythm generator are largely genetically determined. At least 12 genes have been identified on chromosome 5 which, through inhibiting or promoting protein transcription, form a complex negative feedback system which has the properties of an oscillator or clock.

Examples of the genes are period 1–3, clock, BMAL, cryptochrome 1 and 2, and casein kinase.

They are all DNA transcription activators and repressors with complex interactions which result in fluctuating levels of messenger RNA during the day and night.

The clock genes have other functions such as encoding proteins related to cardiovascular function and haem synthesis.

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