Because the brain is only interested in lightness contrast, the eye quickly sensitizes or desensitizes to the amount of surrounding light. If you turn on a light in the middle of the night, your eyes quickly adapt so that everything doesn't appear extremely intense.
The only reason that our eyes don't desensitize to everything is that they are constantly scanning, or moving. Without you even being aware, your eyes are constantly moving back and forth along this web page so that the image reaching the brain is constantly changing. This provides the contrast necessary for us to be able to see.
The eye desensitizes after either a prolonged or intense signal of light. If you stare at a green piece of paper for a long period of time, your eyes will become used to seeing green. When you look away from the green, your eyes will temporarily still be desensitized to the green. As a result, everything that you look at will appear less green than under normal conditions.
The altered image that you see is called a negative afterimage. Because your eyes desensitize there is successive contrast: your perception of something is changed because of something you have just looked at. The longer or more intense a signal is, the longer and more intense your negative afterimage will be.
This effect can be demonstrated for a bright signal using a camera. After a bright camera flash you often see dark spots. This is because your eye has desensitized to light intensity in the area where the flash has hit it, so everything in this area appears less intense or darker.
Negative afterimages can be shown for duration using any of the examples below. Stare at an image for 30-60 seconds then click on it. This will turn the page white; when you stare at the white screen a negative afterimage will form.
An interesting fact about negative afterimages is that they are formed in the retina, not the brain. For this reason, by staring at something while one eye is shut, you can form a negative afterimage in one eye but not the other!