Something wakes you up in the middle of the night, or maybe you're trying to find a light switch or door handle or phone in a dark room. It's happened to all of us. You notice that it's difficult to see for a couple of moments before your vision returns. This process is known as ''dark adaptation'' and it's what makes our eyes see in the dark.
In order for night vision and dark adaptation to occur, several physiological, neurological and biochemical mechanisms must take place behind the scenes. So how does this work? The human eye has, in addition to other cells, rod cells and cone cells, which are found on the retina at the back of the eye. Together they form the sensory layer that helps the eye pick up light and color. Cones and rods exist throughout your retina, except for in the small area called the fovea. It is made up of only cone cells, and its main function involves focusing. You may have heard that the cones help us see color and detail, while rod cells let us see black and white, and are light sensitive.
So, if you want to see something in the dark, like a faint star in a dark sky, you'll be better off if you focus on the area off to the side of it. It works by using the light-sensitive rod cells.
Furthermore, the pupils, the black circles in the middle of your eyes, dilate in low light. It requires less than a minute for your pupil to fully dilate but dark adaptation keeps enhacing your vision for the next half hour and, as you've experienced, during this time, your ability to see in the dark will increase enormously.
Dark adaptation occurs when you first enter a darkened cinema from a well-lit lobby and have a hard time locating a seat. But soon enough, you get used to the situation and see better. You'll experience a very similar phenomenon when you're looking at the stars in the sky. At the beginning, you won't see many. Keep looking; as your eyes continue to dark adapt, the stars will become brighter. While you need several moments to get used to the darker conditions, you'll quickly be able to re-adapt upon your return to bright light, but then the dark adaptation process will have to begin from scratch if you go back into the dark.
This is one reason behind why a lot people have trouble driving their cars at night. If you look directly at the lights of opposing traffic, you are briefly blinded, until that car is gone and you readjust to the night light. A good way to avoid this is to avoid looking right at the car's lights, and learn to use your peripheral vision to observe oncoming traffic at night.
There are numerous things that could potentially lead to trouble with night vision, including: a nutritional deficiency, macular degeneration, cataracts, glaucoma, and others. If you detect that you have difficulty seeing in the dark, call to make an appointment with one of our eye care professionals who will be able to shed some light on the issue.