Picture this: your house loses power and you've got to locate a flashlight or the fuse box. Your eyes normally require a few minutes to adjust to the dark and then you can see again. This process, called ''dark adaptation,'' allows people to see even when there's almost no light.
In order for night vision and dark adaptation to happen, many physiological, neurological and biochemical mechanisms must take place behind the scenes. So how does it really happen? Firstly, let's examine the eye and its anatomy. Your eye has, in addition to other cells, rod cells and cone cells, which are found on the retina at the back of the eye. Together they make up the sensory layer. This is the part that enables the eye to see colors and light. Cones and rods are spread throughout your entire retina, save for the small area opposite the pupil known as the fovea, where there are only cone cells. This section is necessary for detailed sight, such as when reading. As you may know, the details and colors we see are detected by cone cells, while the rods are sensitive to light.
Let's put this all together. Imagine attempting to focus on an object in the dark, instead of focusing right on it, try to look just beside 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.
Furthermore, the pupils dilate in the dark. Your pupil grows to it its biggest capacity in about a minute; however, it takes about 30 minutes for you to achieve full light sensitivity. During this time, your eyes become 10,000 times more sensitive to light.
Here's an example of dark adaptation: when you enter a darkened movie theatre from a well-lit lobby and have trouble finding a seat. After a while, you get used to the dark and see better. You'll experience the same thing when you're looking at the stars in the sky. At the beginning, you won't see many. Keep looking; as your eyes continue to dark adapt, the stars will become easier to see. It takes a few noticeable moments for your eyes to adapt to normal indoor light. If you go back outside, that dark adaptation will vanish in the blink of an eye.
This is actually one reason behind why many people don't like to drive at night. If you look right at the lights of a car heading toward you, you may find yourself briefly blinded, until you pass them and you once again adjust to the night light. A good way to avoid this is to avoid looking right at headlights, and instead, use peripheral vision to observe oncoming traffic at night.
There are a number of conditions that could, hypothetically lead to trouble seeing at night. These include not getting enough Vitamin A in your diet, cataracts, glaucoma, or some other visual obstruction. If you suspect difficulty with night vision, schedule an appointment with one of our eye doctors who will be able to identify and rectify it.