In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels.[1] Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen.[1] In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours[2] in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude.[1] This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity.[1] Light adaptation, in contrast, works very quickly, within seconds.