Nyctalopia (/ˌnɪktəˈloʊpiə/; from Ancient Greek νύκτ- (núkt-) 'night' ἀλαός (alaós) 'blind, invisible' and ὄψ (óps) 'eye'),[1] also called night-blindness, is a condition making it difficult or impossible to see in relatively low light.

The most common cause of nyctalopia is retinitis pigmentosa, a disorder in which the rod cells in the retina gradually lose their ability to respond to the light.

When light falls on rhodopsin, it undergoes a series of conformational changes ultimately generating electrical signals which are carried to the brain via the optic nerve.

Refractive "vision correction" surgery (especially PRK with the complication of "haze") may rarely cause a reduction in best night-time acuity due to the impairment of contrast sensitivity function (CSF) which is induced by intraocular light-scatter resulting from surgical intervention in the natural structural integrity of the cornea.

"[citation needed] Historically, nyctalopia, also known as moonblink, was a temporary night blindness believed to be caused by sleeping in moonlight in the tropics.

[5] In the French language, nyctalopie and héméralopie have inverse meanings, the first naming the ability to see in the dark as well as in plain light, and the second the inability to do so.

[10] The region on horse chromosome 1 to which the Lp gene has now been localized also encodes a protein that channels calcium ions, a key factor in the transmission of nerve impulses.