Sometimes you get ready for bed and turn off the light, but you can’t sleep. You open your eyes and you can’t see a thing. Gradually, the things in the room begin take shape. This process, called ”dark adaptation,” allows people 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. So how does it really work? Let’s examine the eye and its anatomy. The retina is a layer of cells at the back of the eye. The section of the retina across from the pupil that is responsible for sharp focused vision is called the fovea. The retina is made up of rod-shaped and cone-shaped cells. The rods are able to function more efficiently than cone cells in low light conditions. Those cells are not found in the fovea. What’s the functional difference between these two cell types? Basically, details and colors we see are detected by the cones, while rod cells allow us to see black and white, and are light sensitive.
Let’s put this all together. Imagine you want to see something in the dark, like a distant star in a dark sky, instead of looking right at 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.
Also, the pupils dilate in low light. It takes less than a minute for the pupil to completely enlarge; however, it takes approximately half an hour for the eyes to fully adapt.
Dark adaptation occurs when you walk into a darkened movie theatre from a bright area and have a hard time finding a seat. But after a few minutes, you get used to the situation and see better. You’ll experience a very similar phenomenon when you’re looking at stars at night. At first you can’t see very many. Keep looking; as your eyes continue to dark adapt, the stars will gradually appear. It’ll always require a few moments until your eyes fully adapt to normal indoor light, but if you go back into the brightness, that dark adaptation will disappear in a moment.
This explains one reason behind why a lot people have difficulty driving at night. When you look right at the lights of an oncoming car in traffic, you may find yourself briefly unable to see, until that car passes and your eyes once again adjust to the night light. A good way to avoid this is to avoid looking right at headlights, and learn to use your peripheral vision in those situations.
If you’re finding it challenging to see when it’s dark, book a consultation with your eye doctor who will explore the reasons this might be happening, and rule out other and perhaps more serious reasons for poor night vision, like macular degeneration or cataracts.