Something wakes you up in the middle of the night, or you're searching for the light switch or door in a dark room. It's happened to all of us. A number of minutes pass before you are able to distinguish between the objects in the room and the darkness around you. 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. But how does this work? The human eye absorbs photons via rod cells and cone cells, which are found at the back of the eye; or, to be precise, on the retina. Together they make up the sensory layer that enables the eye to detect light and color. Cones and rods are spread throughout the entire retina, with the exception of the small area called the fovea, where there are only cone cells. That part provides detailed sight, such as when reading. You might already be aware that the cones enable us to see color and detail, and rod cells let us see black and white, and are light sensitive.
Considering these facts, if you're looking at an object in the dark, like a dim star in a dark sky, instead of focusing right on it, try to look just beside it. By looking to the side, you use the rods, which work better in the dark.
Furthermore, the pupils, the black circles in the middle of your eyes, dilate in response to darkness. It requires fewer than sixty seconds for the pupil to completely dilate but it takes approximately 30 minutes for you to achieve full light sensitivity.
You'll experience dark adaptation when you first enter a dark theatre from a bright area and struggle to find a seat. But soon enough, you get used to the situation and see better. You'll experience a very similar feeling when you're looking at stars at night. Initially, you can't see very many. Keep looking; while you dark adapt, millions of stars will gradually appear. Despite the fact that your eyes require a few noticeable moments to adapt to the darker conditions, you'll always be able to re-adapt to exposure to bright light, but if you go back into the darker setting, your eyes will need time to re-adjust again.
This explains why a lot people have trouble driving at night. When you look right at the headlights of opposing traffic, you are momentarily unable to see, until that car passes and your eyes readjust to the night light. To prevent this, don't look right at the car's lights, and learn to use peripheral vision in those situations.
There are numerous things that could be the cause of decreased night vision. These include diet-related vitamin deficiencies, macular degeneration, cataracts, glaucoma, and others. If you suspect issues with night vision, book an appointment with one of our eye doctors who will be able to get to the source of the problem.