You're ready for bed and turn out the lights, but you can't fall asleep so you open your eyes and find yourself surrounded by darkness. It takes a few minutes for your eyes to adjust to the darkness. This process, known as dark adaptation allows our eyes to adjust to low light settings and involves a combination of biochemical, physical and neural mechanisms.
So how do your eyes adapt to low light? Let’s start by exploring some eye anatomy. The human eye has rod cells and cone cells on the retina at the back of the eye, which form the sensory layer that enable the eye detect light and color. These cells are distributed evenly throughout the retina except for the small area opposite the pupil known as the fovea, where there are only cone cells. You may have heard that the “cones perceive color” and detail, while the rods help us see “black and white” (and detect movement). This is significant because the rods, which are more sensitive to light, are not on the fovea, which is used for focusing on an object. So when you are trying to see something in low light, don't look directly at it. By using your peripheral vision, you are using more rod cells, which work much better in low light situations. The pupils also dilate in the dark, but the pupil reaches its maximum size within a minute. However, dark adaptation improves over approximately 30 minutes and sensitivity to light may improve by a factor of 100,000 or more.
Dark adaptation occurs when you first enter a darkened theatre from a bright lobby and have trouble finding a seat. After a while, you get used to the dark and see better. Also try looking at the stars in the sky. At first you won't see any, but keep looking. As you dark adapt they will gradually appear brighter over half an hour. While it takes a few noticeable moments for our eyes to get used to the dark, dark adaptation of the rods can be lost in a flash of exposure to bright light. This explains why, when driving at night, if you look directly at the headlights of an approaching vehicle, you are momentarily blinded until the car passes and your eyes once again adjust to the night light. To avoid this, don’t look directly at oncoming cars or lights and learn to use your peripheral vision to guide you.
If you find it increasingly difficult to see at night or in the dark, sometimes it is from a nutritional deficiency, or cataracts, or other obstruction to vision. Schedule an appointment with your eye doctor who will be able to shed some light on why this is happening. Otherwise, enjoy looking at the stars.