You wake suddenly, in the middle of the night, or maybe you’re looking for a light switch or door handle or phone in the dark. These sorts of things happen to us all the time. Your eyes normally require a few minutes to adjust to the dark and then the your surroundings come back into view. This process, called ”dark adaptation,” causes people to see even when there’s very little light.
A person with a healthy set of eyes probably takes night vision – and the biochemical, physical and neural mechanisms – for granted. So how does this work? The retina is a layer of cells at the back of the eye. The portion of the retina directly across from the pupil that is responsible for the point of focus is called the fovea. The retina comprises cone cells and rod cells, named for their respective shapes. The rods are able to function better than cone cells in low light conditions but those cells are not found in the fovea. What’s the functional difference between these two cell types? In short, cones enable us to see color and detail, and the rods let us see black and white, and are light sensitive and detect movement.
So, if you’re struggling to view something in the dark, like the dresser in your darkened room, you’ll be better off if you look at the area off to the side of it. Since there no rods in the fovea, you’ll see better if you avoid using it when it’s dim.
Another process your eye undergoes is pupil dilation. It requires fewer than sixty seconds for the pupil to completely enlarge; however, it takes about 30 minutes for the eyes 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 leave a bright area and enter a dim one, for instance, when you go inside after being out in the sun. Even though you need several moments to begin to see in the dark, you’ll always be able to re-adapt upon your return to bright light, and this resets any dark adaptation that had developed where it was darker.
This explains one reason behind why many people prefer not to drive when it’s dark. If you look at the headlights of opposing traffic, you are briefly unable to see, until that car is gone and your eyes once again adjust to the night light. A good way to prevent this sort of temporary blindness is to avoid looking right at the car’s lights, and learn to try to allow your peripheral vision to guide you.
If you’re struggling to see when it’s dark, call us to schedule an appointment with your eye care professional who will confirm that your prescription is up to date, and eliminate other reasons for decreased vision, such as cataracts and macular degeneration.