Figure 1.

The schematic diagram shows the components and the outline of organization of circadian timing system in humans.

Zeitgebers (light, temperature, and social cues) provide the environmental information to the suprachiasmatic nucleus (SCN; ‘master oscillator’) through melanopsin-expressing retinal ganglion cells (RGCs), which are intrinsically photosensitive (ipRGCs). From the SCN, the information passes to the peripheral clocks through humoral and neural molecules located in various organs including liver, skeletal muscle, pancreas, heart, and kidney. By this way, numerous biochemical, molecular, cellular and behavioral processes are scheduled and coordinated by the circadian clock system.