Night vision is the ability to see in low light conditions. There are a few main differences between daytime vision and night vision:

• The pupils become larger and the eye lets in more light when it’s dark outside.
• A different, more sensitive kind of cell in the eye — rod cells — collects the light for night vision.
• Night vision is mostly or completely in black and white. Color vision is poor in very low light conditions.

What to Do About Night Vision Problems

If you notice that your night vision is getting worse, you should see an ophthalmologist for a full exam. Decreasing night vision can be a sign of serious eye conditions, a temporary side-effect of other conditions or a natural part of aging.

There are no home treatments or exercises that can improve night vision. Prompt medical treatment of any underlying conditions will preserve as much vision as possible. Your doctor can help figure out what’s best for you.

How Do We See In the Dark?

In darkness, the iris opens wider, making the pupil larger. This dilation lets more light into the eye, so more light reaches the retina.

The retina, the light-sensitive tissue at the back of the eye, is covered in two kinds of photoreceptor cells: rods and cones. The cone cells give us the color vision that we have in brighter light. Rod cells are much more sensitive in low light, but only give black and white vision.

In dark conditions, most of the rods in the retina are turned on, but only a few cones are working. That’s why, when it’s dark, vision is all or mostly in black and white. Cones begin to turn on and contribute some color vision when the amount of light approximates what you’d see on a starry night.

If the rods stop working due to disease or injury, a person can lose night vision and won’t be able to see at all in the dark.

What Problems Do People Have with Night Vision?

Some of the most common problems people have seeing at night are:

• Trouble seeing objects that aren’t nearby
• Blurry vision
• Glare or halos around lights

Trouble seeing at night can increase the risk of falling and can make it hard to drive safely.

As the pupil gets bigger in the dark, some people become more nearsighted than they are in bright light. This night myopia can make distant objects appear fuzzy. This condition may be a sign of a need for glasses or a change to your glasses’ or contacts’ lens power.

Some of the diseases and conditions that can affect night vision are:

• Cataracts. Trouble seeing or driving at night can be one of the first symptoms of a cataract. Cataracts can make vision blurry or dim and can increase glare, halos or streaks from headlights and streetlights.
• Glaucoma. Glaucoma first attacks peripheral vision before harming central vision. Both daytime and nighttime vision are affected as parts of the retina stop working.
• Macular degeneration. As the retina changes from macular degeneration, distortion and blind spots can appear in both daytime and nighttime vision.
• Retinal diseases including retinitis pigmentosa. Diseases that directly affect the retina often harm low-light vision first because there are far more rod cells in the retina. Rod cells also cluster more around the edge of the retina, where damage may start.
• Vitamin A deficiency. Night blindness (nyctalopia) is one of the first signs of vitamin A deficiency. Vitamin A deficiency reduces the production of rhodopsin, the pigment that your rods need to see in low light.
• Congenital night blindness. Several genetic conditions can reduce or block night vision.
• Refractive error. If you need glasses or contacts, or need a new prescription, your night vision will be blurry just like your daytime vision.
• Refractive surgery (Lasik, PRK, etc.). Changes in the shape of the cornea can affect how light is bent and cause more glare or halos around lights at night.

How is Night Vision Tested?

To assess night vision, your ophthalmologist may ask you questions, look at your eyes and test your vision. One common test is the Pelli-Robson Contrast Sensitivity Chart. It is like the Snellen eye test letter chart, but with letters in different shades of grey. This test measures how well you can see the contrast between white paper and light grey shapes.

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Ravi D. Goel M.D. is a cataract surgeon & comprehensive ophthalmologist at Regional Eye Associates in Cherry Hill, NJ.  He is also a clinical instructor at Wills Eye Hospital.  His patient-friendly YouTube cataract surgery educational videos are here.