You're settled down for bed, so you turn out the lights, but you can't fall asleep. You open your eyes and it's all dark. You notice that it's difficult to see for a few moments before your eyes adjust. This process, "dark adaptation," allows our eyes to adjust to the dark.
In order for night vision and dark adaptation to work, many physiological, neurological and biochemical mechanisms must take place behind the scenes. Let's have a look at how this works. Your eye absorbs photons via two kinds of cells: cones and rods, which are found at the back of the eye; or, to be precise, on the retina. Together they form the sensory layer that helps the eye pick up light and color. Cones and rods are found throughout your retina, save for the small area opposite the pupil known as the fovea, where there are only cone cells. The fovea is the part used for detailed sight, such as when reading. What's the functional difference between these two cell types? Basically, cones contribute to color vision, while the rods are sensitive to light and detect movement.
Now that you know some background, let's relate it to dark adaptation. If trying to get a glimpse of an object in the dark, like a distant star in the night sky, instead of looking directly at it, try to look just beside it. It works by utilizing the light-sensitive rod cells.
Another process your eye undergoes is pupil dilation. It requires less than a minute for the pupil to fully dilate; however, it takes approximately half an hour for you to fully adapt.
Dark adaptation occurs when you enter a darkened movie theatre from a well-lit lobby and have trouble finding somewhere to sit. But after a couple of minutes, your eyes adapt to the situation and before you know it, you can see. You'll experience a very similar feeling when you're looking at the stars in the sky. At the beginning, you probably won't be able to actually see that many. Keep looking; as you dark adapt, the stars will become easier to see. It'll always require a few moments until your eyes fully adapt to normal indoor light. If you go back outside, that dark adaptation will disappear in a flash.
This is actually one reason behind why so many people have trouble driving at night. When you look at the lights of a car heading toward you, you may find yourself briefly blinded, until you pass them and you readjust to the night light. A helpful way to prevent this sort of temporary blindness is to avoid looking right at headlights, and learn to use peripheral vision to observe oncoming traffic at night.
There are several conditions that could, hypothetically cause difficulty with night vision, including: not getting enough Vitamin A in your diet, macular degeneration, cataracts, glaucoma, and others. If you notice problems with night vision, schedule an appointment with one of our eye care professionals who will be able to locate the root of the problem.