There Goes the Sun – How Your Eyes Adjust to Darkness

Our eyes are constantly adjusting to different light environments throughout the day, from walking inside from the bright sunlight outside, or into a dark room from a lighted one. While we all need light to see, the truth is, we only really need some light to see, and oftentimes we can actually see phenomenally well in very low light.

Considering the fact that we’re heading into that time of year when the night is considerably longer than the day, our ability to see in low light comes in really handy.

Generally speaking, there are two processes at work simultaneously that allow us to see in low light. The first is the pupil of the eye changing its size, and as a result, how much available light your eye can perceive. The second process involves the two different types of photoreceptors located in the retina of your eye, the back part of the eyeball, known as cones and rods.

Bright Light, Tiny Pupils

When there is abundant bright light, the pupil of your eye contracts and becomes very small, allowing only as much light as necessary to reach the back of your eye, where it’s perceived by your retina, from where the information is transferred to your brain and interpreted as images.

On the contrary, when the light is low, your pupils dilate, opening as wide as they can to take in whatever limited light is available. While we’re not able to make out much detail in low light, we are able to see quite a lot as our eyes adjust to the darker environment.

Cones and Rods

The retina of your eye has two different types of photoreceptors called cones and rods. Cones are dynamic cells working all the time there is plenty of light, and are also able to perceive and differentiate colors, whereas rods cannot. Without cones, we would not have color vision. Certain variations in the cones of your eye, variations that are passed down genetically, cause some people to see colors differently than others, also known as color blindness.

Rod cells, on the other hand, step up when there is low light. Being the more sensitive cells of the two, rods can pick up even the smallest amount of light available while your pupils are wide open taking in whatever they can get. Think about all those times you’re walking around at night navigating your environment only by starlight, and maybe not even that much starlight at all! The rods in your eyes are using what little light the stars are providing to make out the general shapes and shadows of the environment. Rods aren’t able to perceive color, so we generally don’t see many colors in darkness, and they can’t discern much detail, so we’re not usually able to read very well in low light, but we can tell where we’re walking and that there’s a street sign on the corner, even if we can’t make out what it says.

The Flip of a Switch

Our eyes are constantly adjusting to different light conditions we move through throughout the day. As conditions change slowly, our eyes can generally keep pace, but when the lights suddenly go out, it takes a few minutes for everything to catch up. Our pupils open wide, our cones start to take a back seat to our rods, and within ten minutes or so, depending on how much light is available, we’re usually able to get along pretty well.