Nyctalopia

Also Night Blindness; A condition that not only worsens night vision but also requires extra time to adjust from brightly lit areas to dim ones. Contrast vision may also be greatly reduced. There are several causes of Nyctalopia:

Congenital:

• Retinitis Pigmentosa (the most common cause), where the Rods in the retina gradually lose their sensitivity, making Nyctalopia often a progressive (worsening) disease.
• Congenital conditions such as X-linked stationary night blindness, where the rods (from birth) either do not work at all, or work very little, but the condition does not get worse.
• Injury
• Malnutrition (e.g. vitamin A deficiency)

There are several claims that Colorblindness is correlated with better or worse night vision. However, there is a lack of evidence either way. Additionally, the proponents of better vision tend to use incorrect assumptions in explaining their claim, either:

• Rods & Cones are often described as enabling “Contrast” & Color vision respectively. Because there is plenty of evidence to show that the Colorblind have slightly better contrast sensitivity, it would follow that the Colorblind have better Rods and therefore better Night Vision. However, a paper from Nature shows that the Colorblind only have better contrast sensitivity in colorful images where Color Normals are ‘distracted’ by colors [Sousa 2020]. In grayscale (such as with night vision), there is no advantage.
• If the Colorblind are missing a type of Cone (only applies to Dichromats), then there is more room for more Rods. However, the missing cones are replaced with other cone types, not with Rods (see Retinal Mosaic).
• They feel they are better when comparing to others. However, Rod-driven vision lacks color vision in all humans and therefore Colorblind individuals do not have a deficiency at night. That lack of a deficiency may seem like a “surplus”.

The opposite problem – the inability to see in bright light – is known as Hemeralopia (Day Blindness) and is a symptom of Achromatopsia.