“How it knows what time it is: - Light enters your eyes - Hits photoreceptors in your retina (rods, cones, and intrinsically photosensitive retinal ganglion cells containing melanopsin) - Melanopsin cells are most sensitive to blue light (480nm) — the wavelength of noon sky - These cells send signals directly to the SCN (via the retinohypothalamic tract) - SCN adjusts the master clock based on light input”