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You're all ready for a good night's sleep, so you flick the light switch, but you can't sleep. You open your eyes and all you see is black. At first you can't see, but gradually the things in the room begin become visible. This process is known as ''dark adaptation''.

A person with a healthy set of eyes probably takes night vision - and the biochemical, physical and neural mechanisms - for granted. So how does this work? The retina is a layer of cells at the back of the eye. The portion of the retina directly across from the pupil that is responsible for 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 are able to function even in low light conditions. They are not found in the fovea. What's the difference between these two cell types? In short, details and colors we see are sensed by the cones, while the rods allow us to see black and white, and are light sensitive and detect movement.

Let's put this all together. Imagine you want to see something in the dark, like a small star in a dark sky, instead of focusing right on it, try to use your peripheral vision. Since there no rods in the fovea, you'll see better if you avoid using it when it's dim.

Also, the pupils dilate in the dark. It requires fewer than sixty seconds for the pupil to completely dilate; however, your eyes will continue to adapt over a half hour time frame.

Here's an example of dark adaptation: when you first enter a darkened cinema from a bright area and have a hard time finding a seat. After a while, you get used to the dark and before you know it, you can see. This same thing occurs when you're looking at the stars in the sky. At first you can't see very many. As you keep staring, your eyes will dark adapt and millions of stars will gradually appear. It takes a few noticeable moments until you begin to adjust to normal indoor light, but if you walk back out into the brightness, that dark adaptation will vanish in a moment.

This is one reason behind why a lot people have difficulty driving at night. When you look right at the headlights of an approaching car, you may find yourself briefly unable to see, until that car is gone and your eyes readjust to the night light. To prevent this, try not to look right at the car's lights, and learn to try to allow your peripheral vision to guide you.

There are numerous conditions that can cause decreased night vision. Here are some possibilities: not getting enough Vitamin A in your diet, macular degeneration, cataracts, glaucoma, and others. Should you begin to detect issues with night vision, schedule an appointment with one of our eye care professionals who will be able to shed some light on the issue.