Why Does Our Vision Adapt to the Dark so Slowly?

All men who go clubbing know it: in the dark, all the girls are pretty. That's another story during daylight. Try the opposite and it will also result tricky: from the outdoors on a bright sunny day into a dark room, we can hardly distinguish anything at first.

But we gradually start to detect the room's contents, a phenomenon named "dark adaptation", which takes 20 to 30 minutes.
We have two types of light-sensitive cells (photoreceptors) in the retina: cones, for day vision, sensitive to changes in brightness even in extremely high levels of illumination and rods, for night vision, that are detectors even in extremely low levels of light but are ineffective in bright light. Rods can react to even a single visible photon, working at the physical limit of light detection.

Both cones and rods slowly increase their sensitivity in a dim surrounding. Cones act quicker, in the first few minutes of "dark adaptation". After that, the slower rods take over, as they are the most sensitive receptor type.

Photons interact in photoreceptors with proteins called opsins (visual pigments). Human cones have three kinds of opsins, sensitive to different light wavelenghts, responsible for color vision. Rods have just a type of pigment named rhodopsin (that's why we see black and white during the night).

Opsins contain a molecule named retinal, derived from vitamin A (that's why shortage of this vitamin causes night blindness.) A photon induces changes in the retinal molecule's hydrocarbon chain, a process called photoisomerization.

This turns on the opsins, which start a series of biochemical reactions leading to shifts in the number of glutamate molecules released by the photoreceptor. This amino acid acts like a neurotransmitter, delivering information about the light stimulation of photoreceptors. Free opsins (that have released their retinal molecules) could be the cause of the sensitivity lack of the retina during the dark adaptation.

In this case, original opsins have to be restored, by combining free opsins with an unactivated retinal.
The rate of retinal supply to the photoreceptors could be the main reason for the relatively slow speed of dark adaptation. This process evolved to adapt to the slow shifts in natural illumination during the sunset, being fitted to changes in natural lighting.

Besides vitamin A shortage, a more common cause in non-developed countries, night blindness can be also genetic, caused by mutations, like the retinitis pigmentosa.

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