The ability of nocturnal animals to function in what human eyes would perceive as complete darkness seems almost magical. Nearly everyone has experienced the sense of panic that sets in shortly after a power outage. With the sudden loss of artificial light, you realize that you can't see a thing and you feel vulnerable. Moving about in the darkness, even in the familiar environment of your own house, is hazardous. The pile of books you left on the floor, the dreaded coffee table leg -- these and other dangers lie in wait for you. Finally, after fumbling around in the dark, you locate the flashlight. Turning it on, you feel your world open up again.
The nighttime world is never closed for nocturnal animals. Unlike humans, these creatures possess adaptations, acquired over millions of years and countless generations, that allow them to see even when the night sky is at its darkest. In most ways, a nocturnal animal's eyes are very similar to our own. Like cameras, they possess a lens, an exposure meter, and automatic focus. Nocturnal eyes differ from ours, however, in their ability to maximize the amount of light they receive, as well as in their sensitivity to that light.
Most nocturnal animals have eyes that are inordinately large relative to their body size. This adaptation, in conjunction with the ability most nocturnal creatures have to dilate their pupils far wider than humans can, maximizes the amount of light that can enter the eye. Most nocturnal creatures also have a reflective membrane inside their eyes, called a tapetum. This membrane helps the eye conserve the light it takes in by reflecting it back toward the retina before it escapes the eye completely.
In addition, the retinas of nocturnal eyes are equipped with a high percentage of one of the two types of light-receptor cells. These cells, called rods, are extremely light sensitive. In fact, their sensitivity is about 500 times greater than the sensitivity of the other type of receptor cells, called cones. Only one photon, the smallest measurable unit of light, is required to stimulate a rod into sending a signal to the brain.
There is a price for this increased sensitivity, however. Rods do not provide the high resolution and color vision that cones do, and so many animals that can see better at night than humans do typically see more poorly during the day than we do. This trade-off between light sensitivity and vision quality is the compromise that has accompanied the evolution of night vision. However, there must certainly have been a strong evolutionary benefit to some groups of animals of being active at night. Whether night activity allowed these animals to escape desert heat during the daytime or avoid competition with or predation by creatures that were dominant during the day, the behavior persisted, and adaptations that aided their nighttime vision continued to evolve.