Even if you’re not squinting to see the clock, it doesn’t mean your eyes are in the clear: Many people have vision problems and don’t even know it, a new study published in Optometry and Vision Science suggests.

The researchers found that 58 percent of people with no known vision symptoms when they went in for a routine eye exam actually had at least one significant change noted by their eye doctor.

In the majority of the cases, the diagnoses were relatively minor, like the need for glasses or contacts or an updated prescription for their eye gear.

Still, walking around with an outdated prescription isn’t exactly harmless: You’re more likely to experience eyestrain, headache, and dizziness, according to lead researcher Elizabeth Irving, Ph.D., professor of optometry and vision science at the University of Waterloo.

When that happens, it can range from disorienting to dangerous—if you’re behind the wheel when it kicks in, for example.

What’s more, the researchers classified 14 percent of the changes seen on exams as “new critical diagnoses,” which means conditions that could result in either partial or full loss of vision.

The most common of these critical diagnoses was glaucoma.

Glaucoma—a group of diseases that damage your eye’s optic nerve—usually occurs without many obvious symptoms at first. And when symptoms like distorted vision or partial vision loss do crop up, they usually appear gradually.

That means it may take awhile for you to notice that something’s wrong.

And if you wait until the problem becomes severe to get it checked out, it may be too late, she says. Like many other conditions, glaucoma treatment options and outcomes are potentially better if you catch it early.

While glaucoma is most common over the age of 60, people of any age can get it—and your risk of developing it earlier grows if you have other health conditions like diabetes or high blood pressure, or if you have a family history of glaucoma.

So How Often Do You Need an Eye Exam?

Talk to your eye doctor (or primary care provider, if you don’t have an eye doc yet) to determine how often you should get your eyes checked, since certain health conditions like diabetes or high blood pressure can raise your risk for vision problems, Irving says.

But if you’re a healthy guy, seeing an eye doctor for a routine checkup every 2 to 3 years is a good rule of thumb to protect your eyes, she says.

If you’re already experiencing problems with your vision, though—say, eye pain, irritation, double vision, light flashes, black spots, difficulty reading, or eye fatigue—set up an appointment ASAP.

This can help your doctor diagnose any ailments at an earlier stage when they are easier to treat, she says.