[Rhodopsin](https://en.wikipedia.org/wiki/Rhodopsin) is a protein photopigment used by the rod cells of the retina to detect gray-scale brightness and contrast, and is the key to adaptive night vision. Intense light causes the protein to decompose reducing its sensitivity in bright light (when we rely more heavily on our color-sensing cone cells anyway). Darkness allows the protein to regenerate in a process called “dark adaptation”, allowing the eye to see in low light conditions, but with reduced color discernment.
This process allows us to be able to see fairly effectively in both bright and dim light, but it takes time for our vision to adjust to sudden shifts in brightness in either direction.
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