Fig. 1

Light input is received daily by specialized photoreceptor cells in the retina, the intrinsically photosensitive retinal ganglion cells (ipRGCs) and transmitted via the retino-hypothalamic tract to the central clock located in the suprachiasmatic nucleus (SCN) entraining it to the external light dark cycle(Gooley et al. 2001). Then, the SCN synchronizes all tissues and cells via humoral or neural signals maintaining coordination with each other. Besides this synchronization, each tissue and cell has its own molecular clock, the peripheral clocks. Zeitgebers other than light, such as food, temperature or social cues, can entrain/synchronize or strenghten the robustness of entrainment/synchronization of both the SCN and peripheral clocks. This will result in the temporal organization of the many functions and behaviours that oscillate daily in most organisms on the planet