page_title: Adaptation (eye)
section_title: Insufficiency
hierarchical_section_title: Adaptation (eye) / Insufficiency
caption_attribution_description: Nederlands: Zicht op de weg. Links: goed nachtzicht. Rechts: nachtblind.
context_page_description: 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. 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. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours
  in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.
context_section_description: Insufficiency of adaptation most commonly presents as insufficient adaptation to dark environment, called night blindness or nyctalopia. The opposite problem, known as hemeralopia, that is, inability to see clearly in bright light, is much rarer.
  The fovea is blind to dim light (due to its cone-only array) and the rods are more sensitive, so a dim star on a moonless night must be viewed from the side, so it stimulates the rods. This is not due to pupil width since an artificial fixed-width pupil gives the same results.
  Night blindness can be caused by a number of factors the most common of which being vitamin A deficiency.  If detected early enough nyctalopia can be reversed and visual function can be regained; however; prolonged vitamin A deficiency can lead to permanent visual loss if left untreated.
  Night blindness is especially prominent in developing countries due to malnutrition and therefore a lack of vitamin A in the diet.  In developed countries night blindness has historically been uncommon due to adequate food availability; however, the incidence is expected to increase as obesity becomes more common.  Increased obesity rates correspond to an increased number of bariatric surgeries, causing malabsorption of vitamin A in the human body.
caption: Effect of night blindness. Left: good night vision. Right: nightblind.
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