[INTRO] [What happens if you leave your contacts in for too long?] If you wear contact lenses, your doctor probably told you not to wear them for too long, and there are a lot of good reasons for that.

A student in England, for instance, tried to take her contacts out with dry eyes after leaving them in for 10 hours and ended up ripping off part of her cornea. Another student in Taiwan (why is it always students?) left contacts in for six months, which created the perfect environment for bacteria and amoebas to burrow into her eye.

Ugh. Just got to get the nice outside-of-your-eyes lenses. These are good.

So even though contacts are generally safe to wear, they do come with lots of biological risks if you don't follow your doctor's instructions. Contact lenses work like glasses do: bending light, so it focuses properly on the photoreceptors at the back of your eye, which send information to your brain. Contacts sit right on top of your cornea.

That's the outermost layer of transparent tissue that helps focus light entering your eye and protects everything else from things like dust, germs and UV radiation. There are two main dangers to leaving your contacts in for too long. For one, the cells on your cornea are alive and need to stay wet, so you have a built-in moisturizing system.

You have glands that coat your eyes with tears, which are a mix of water and things like oils, nutrients, immune cells, and mucus-related proteins, but contacts can interfere with this tear layer by soaking up moisture or keeping it from getting to your eyes' cells. Dry eyes can get really irritating, and could lead to corneal damage. And second, leaving contacts in for too long can increase your risk of getting an eye infection from some nasty microorganisms.

Contacts that have been sitting on your cornea for a while can cause physical scratches that damage your cells or reduce the flow of oxygen that cells need to survive, and then if you didn't wash your contacts well enough with a disinfecting solution, any bacteria, viruses, fungi or parasites hanging around can get into the damaged cornea tissue. With enough time these, these pathogens can multiply and cause a full-blown cornea infection, generally called keratitis. Keratitis usually starts with Inflammation and pain as your body tries to fight it off, but it can get really bad without treatment.

If your cornea gets too damaged, you can even lose your eyesight. But even if you are really good about following instructions, scientists have discovered that using contacts regularly for long periods of time—like more than five years—can affect your eyes, too. Several studies have found that long-term contact use can reduce the thickness of your cornea.

It's not necessarily bad, but enough of a shape change could affect your vision or prevent you from getting laser eye surgery safely. We're not exactly sure why this thinning happens, but it might have something to do with the reduced amounts of oxygen affecting how the tissue grows. So you might wear contacts for lots of reasons like convenience, appearance or to have closer to normal vision.

Even though leaving them in for too long can get really bad, the risks are generally small as long as you are careful to do what your doctor tells you to do. Thanks to our Patreon patrons for asking, and if you want to learn more about vision science, you could check out our video that explains how glasses don't really ruin your eyesight. [OUTRO]