Let's say there's a power outage and you've got to locate a flashlight or the fuse box. You notice that it's difficult to see anything for several moments and then your eyes adjust. This is called ''dark adaptation''.
In order for night vision and dark adaptation to occur, several physiological, neurological and biochemical mechanisms must take place behind the scenes. Let's talk about how your eye actually operates in these conditions. The human eye absorbs photons via two kinds of cells: cones and rods, which are found on the retina at the back of the eye. Together they make up the sensory layer. This is the part that gives your eye the ability to see colors and light. Cones and rods are found throughout your retina, except for in 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, for example when reading. As you may know, the cones contribute to color vision, and the rods let us see black and white, and are light sensitive and detect movement.
Now that you know some background, let's relate it to dark adaptation. If you want to see something in the dark, like a small star in the night sky, instead of focusing right on it, try to look just beside it. That way, you're avoiding the use of the fovea, which only has cells that are less sensitive to low light.
In addition to this, the pupils dilate in low light. It requires approximately one minute for your pupil to fully enlarge; however, your eyes will keep getting used to the dark over a 30 minute time frame and, as everyone has experienced, during this time, your ability to see despite the darkness will increase remarkably.
Dark adaptation occurs when you go from a very light-filled area to a dim area for instance, when you go inside after being out in the sun. It takes a few noticeable moments until your eyes fully adapt to regular indoor light. If you go back outside, those changes will disappear in a moment.
This explains why a lot people prefer not to drive at night. If you look at the headlights of a car heading toward you, you are momentarily blinded, until that car is gone and you once again adjust to the night light. A helpful way to prevent this sort of temporary blindness is to avoid looking directly at the car's lights, and learn to try to allow peripheral vision to guide you.
There are numerous conditions that could potentially cause difficulty with night vision, including: a nutritional deficiency, macular degeneration, cataracts, glaucoma, and others. If you detect that you experience problems with seeing at night, call to make an appointment with one of our eye doctors who will be able to shed some light on why this is happening.