Morning light on bare skin and open eyes quietly runs two separate programs. Ultraviolet B photons strike epidermal cells and convert 7‑dehydrocholesterol into cholecalciferol, the raw material for vitamin D. At the same moment, short‑wavelength light hits intrinsically photosensitive retinal ganglion cells, which fire directly to the suprachiasmatic nucleus, the brain’s master circadian pacemaker...More