Med thumb contact lens

If you’re among the more than 30 million American adults who wear contact lenses, there’s a good chance you’ve fallen asleep with those little plastic cylinders still suctioned to your corneas. Maybe you passed out during a Netflix binge, or maybe you ran out of saline. Maybe you just got drunk and forgot. Usually, it’s a harmless mistake.

Except when it’s not.

Sleeping with your contacts is playing eye-care roulette, explains Dr. Mark Eltis, an Ontario-based optometrist and clinical professor who focuses on eye disease. Though most people who don’t remove their lenses before bed will wake up without pain or pus, a small percentage will develop microbial keratitis, an unsavory and potentially blinding bacterial infection that can ransack eyes in just one night. Each year, microbial keratitis affects at least 30,000 people in the U.S.

Why is sleeping in contacts so bad?

Eye-care experts aren’t quite sure. For years, doctors believed that keratitis developed because closed (sleeping) eyes blocked oxygen from reaching the corneas. Though the original contact lenses were hard, soft lenses hit the market in the 1970s and quickly took over. Somewhat counterintuitively, soft lenses let less oxygen pass through. Without oxygen, eyes become dry and infection-prone.

Or that was the thought. The oxygen theory started to fall apart about 15 years ago, when continuous-wear lenses became all the rage and, shortly thereafter, caused all the infections.

But doesn’t “continuous wear” mean wearing continuously?

Continuous-wear lenses are soft, silicone-based lenses with a tremendously high oxygen content. In the late 90s, manufacturers touted continuous-wear as a 24/7 lens, and doctors prescribed them in droves.

Unfortunately, these patients suffered just as much keratitis from sleeping with their continuous-wear lenses. Though the FDA never pulled continuous-wear from the shelves, cautious doctors stopped prescribing them for overnight use.

As for figuring out the root cause of keratitis, it was back to the drawing board.

Experts came up with a few new theories, but Dr. Eltis says there’s no consensus yet. Some research suggests that our immune systems take a break when we’re sleeping, thus increasing susceptibility to infection.

Sometimes I run out of contact lens solution. What then?

Don’t run out. Or, carry a spare pair of lenses in your purse or messenger bag.

If that’s not an option, Dr. Eltis recommends daily disposable lenses for people — particularly young adults — who make game-time decisions to sleep away from home. It’s better to throw out your lenses than to sleep in them.

The obvious alternative, and one we’ve all improvised at one time, is using plain water for storage. This is no better than sleeping in them, says Dr. Eltis. (He declined to say which terrible idea is less terrible.) Storing your contacts in water can actually cause a different, non-bacterial type of keratitis that tends to do less immediate damage, but is also harder to diagnose and treat.

Keratitis can develop in a single night, and may result in vision loss.

What’s the deal with “nighttime-only lenses”?

They’re in a different ball park. Nighttime lenses perform an entirely distinct function. People with mild prescriptions can wear them at night, and only at night, to reshape their corneas and temporarily improve their vision. Then, during the day, they can see without glasses or contacts.

Nighttime lenses, are more of a compelling idea than a useful product, says Dr. Eltis. While they’re designed to admit more oxygen, nighttime lenses still can and do lead to occasional cases of keratitis.