Fig. 1.

Schematic illustrating key components of the circadian system. The circadian system consists of a network of circadian oscillators. The central clock is located in the suprachiasmatic nucleus (SCN). Neurons in this cell population receive light information from melanopsin-expressing retinal ganglion cells found in the retina. The axons of these ganglion cells make a direct synaptic connection onto cells in the SCN. These SCN neurons integrate this photic information with other timing cues to generate robust circadian oscillations that are synchronized to the environment. Signals from the SCN travel out via the hypothalamic-pituitary-adrenal (HPA) axis, as well as through the autonomic nervous system (ANS), to coordinate and regulate the independent circadian oscillations found throughout the body.