It's happened to all of us: there's a power outage and you've got to find a flashlight or the fuse box. A number of minutes pass before you begin to recognize familiar things in your surroundings. This process, ''dark adaptation,'' causes us to see even when there's very little light.
Many people don't know that night vision is dependent upon several physical, neural and biochemical mechanisms. So how does this work? The retina is a layer of cells at the back of the eye. The area of the retina behind the pupil which produces sharp focused vision is called the fovea. The retina is made up of cone cells and rod cells, named for their respective shapes. The rod cells have the capacity to function better than cone cells in low light conditions but those cells are absent from the fovea. What's the functional difference between rods and cones? Basically, cones contribute to color vision, while rod cells are sensitive to light.
How does this apply to being able to see in the middle of the night? When you want to see something in the dark, like the edge of the last stair in a dark basement, it's better to look at the area off to the side of it. If, on the other hand, you focus on the object itself, you'll use the fovea, which is made up of cone cells that are less responsive in low light conditions.
Another part of the process is pupil dilation. Your pupil reaches its biggest capacity within 60 seconds but it takes about half an hour for you to achieve full light sensitivity and, as everyone has experienced, during this time, your ability to see in the low light setting will increase enormously.
Here's an example of dark adaptation: when you first enter a darkened cinema from a bright area and have trouble finding a seat. But after a few minutes, your eyes get used to the dark and before you know it, you can see. This same thing occurs when you're looking at stars at night. Initially, you won't see many. Keep looking; while you dark adapt, millions of stars will become visible. Despite the fact that your eyes require several moments to begin to see in the darker conditions, you'll quickly be able to re-adapt to exposure to bright light, and this resets any dark adaptation that had developed where it was darker.
This is why a lot people prefer not to drive when it's dark. When you look right at the ''brights'' of an approaching car, you are momentarily unable to see, until that car is gone and your eyes once again adjust to the night light. A good way to prevent this sort of temporary blindness is to avoid looking right at the car's lights, and learn to try to allow your peripheral vision to guide you.
There are several conditions that may cause decreased night vision. Here are some possibilities: diet-related vitamin deficiencies, macular degeneration, cataracts, glaucoma, and others. If you suspect issues with night vision, schedule an appointment with one of our eye care professionals who will be able to shed some light on why this is happening.