Figure 2.

Schematic representation of the light entrainment and intercellular signal transmissions. The mammalian circadian systems are most sensitive to the light and dark cycles around the 24-h a day. The light serves as the most inevitable external zeitgeber to entrain the mammalian circadian rhythms. Usually, the photic signals from the light sources are received by the ipRGCs and reach the master clock, located at the SCN under the hypothalamus of the brain via RHT. In SCN, the primary oscillations are produced and get transmitted to the cellular-molecular level of distal peripheral tissues across the organism’s body. Thus, the different tissue-specific circadian outputs are produced. In addition, the locally regulated peripheral rhythms are integrated together to maintain circadian homeostasis, keeping a synchronization to the master clock.