Life on Earth is profoundly impacted by various environmental cycles of defined periodicities.

Organisms cope with them with the help of biological clocks that closely match the period of the cycle they track1. 

Circadian (~24 hour [h]) clocks allow organisms to optimize their physiology and behavior with the time-of-day. For example, most of us humans sleep during the night and are active during the day. 

Our sleep/wake and feeding/fasting cycles, as well as many other rhythmic physiological processes, are coordinated by circadian pacemaker neurons located in the suprachiasmatic nucleus (SCN) of the hypothalamus.

These neurons synchronize cell-autonomous circadian clocks present throughout our body, which regulate locally rhythmic gene expression2.

At the molecular level, the core circadian molecular clock in animals is a negative transcriptional feedback loop that comprises the heterodimeric transcriptional activator

Source: Nature