A surprising new study finds that people who leave their contact lenses in at night because they don’t feel like taking them out may be in for a nasty surprise.

A new survey by the Centers for Disease Control and Prevention has found that a third of all respondents have had to go to the doctor because of red or painful eyes, according to an NBC News report.

Half of all wearers admitted they often slept while wearing the lenses, which is not recommended. And about 55 percent said they topped off the solution rather than completely change it and then rinsing the case. Further, 61 percent admitted to swimming while wearing the lenses.

And that’s to say nothing of teenagers, who weren’t included in the survey. Teenagers have been found to be even less likely to practice good hygiene when it comes to contact lenses.

People often don’t take proper care of their contact lenses, with many admitting to rinsing them with tap water rather than the proper solution. More than 90 percent of people who had rigid or hard lenses admitted to sometimes rising them with tap water.

Although tap water is perfectly safe to drink, it isn’t sterile and there are microorganisms that contaminate the cases and lenses themselves, leading to eye infections.

It results in lots of visits to the doctor for a condition known as keratitis, which is an inflammation of the cornea. Estimates indicate that the U.S. spends $175 million on these problems each year. Poor contact lens hygiene is the number one cause of keratitis.

Want to avoid it? Practice the basics. Don’t sleep, shower, or swim while wearing them. Don’t touch contacts with dirty hands. And always use disinfection solution and rinse the case.