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Animals renowned for their outstanding night vision include owls, cats and even the dung beetle.

But humans? Not so much.

Over time, many people suffer from night blindness, also known as nyctalopia. This condition makes seeing in dim or dark settings difficult because your eyes cannot adjust to changes in brightness or detect light.

Night blindness is especially problematic and dangerous when driving. If you have it, your eyes cannot adjust between darkness and the headlights of oncoming vehicles; other cars may appear out of focus; and your depth perception becomes impaired, which makes it difficult to judge distances.

Night blindness may also affect your sight at home by making it hard for your vision to quickly adjust to a dark room after turning off the lights.

“This can cause people to bump into furniture or trip and suffer an injury,” said Dr. Isabel Deakins, an optometrist with Harvard-affiliated Massachusetts Eye and Ear.

Why do our eyes do this?

The ability to see in low-light conditions involves two structures in the eye: the retina and the iris.

The retina contains two types of light-detecting cells: cones and rods. Cones handle color vision and fine details, while rods manage vision in dim light.

The iris is the colored part of your eye. It contains muscles that widen or narrow the opening of your pupil to adjust how much light can enter your eyes.

If your irises don’t properly react, the pupils can dilate and let in too much light, which causes light sensitivity and makes it hard to see in bright light.

Or your pupils may remain too small and not allow in enough light, making it tough to see in low light.

What causes night blindness?

Night blindness is not a disease but a symptom of other conditions. “It’s like having a bruise on your body. Something else causes it,” Deakins said.

Several conditions can cause night blindness. For instance, medications such as antidepressants, antihistamines and antipsychotics can affect pupil size and thus how much light enters the eye.

Eye conditions that can cause night blindness include:

• glaucoma, a disease that damages the eye’s optic nerves and blood vessels,
• cataracts, cloudy areas in the lens that distort or block the passage of light through the lens, and
• dry eye syndrome.

However, one issue that raises the risk of night blindness that you can’t control is age.

“Our eyes react more slowly to light changes as we age, and vision naturally declines over time,” Deakins said. “The number of rods in our eyes diminish, pupils get smaller, and the muscles of the irises weaken.”

Can you reduce night blindness?

If you notice any signs of night blindness, avoid driving and get checked by an eye care specialist, such as an optometrist or ophthalmologist.

An eye exam can determine if your eyeglass prescription needs to be updated.

“Often, a prescription change is enough to reduce glare when driving at night,” Deakins said. “You may even need separate glasses with a stronger eye prescription that you wear only when driving at night.”

Adding an anti-reflective coating to your lens may help to cut down on the glare of the headlights of an oncoming car.

However, skip the over-the-counter polarized driving glasses sold at many drug- stores. “These may help cut down on glare, but they don’t address the causes of night blindness,” Deakins said.

An eye exam will also identify glaucoma or cataracts, which can be treated.

Glaucoma treatments include eyedrops, laser treatment or surgery. Cataracts are corrected with surgery to replace the clouded lens with an artificial one. Your eye doctor can also help identify dry eye and recommend treatment.

Ask your primary care clinician or a pharmacist if any medications you take may cause night blindness. If so, it may be possible to adjust the dose or switch to another drug.

Ways to make night driving safer

You also can take these steps to make night driving safer:

• Wash the lenses of your glasses regularly. And take them to an optician to buff out minor scratches.
• Keep both sides of your front and rear car windshields clean so that you can see as clearly as possible.
• Dim your dashboard lights, which cause glare, and use the night setting on your rearview mirror.

Matthew Solan is executive editor at Harvard Men’s Health Watch.

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