Overnight contacts, known as orthokeratology or ortho-k lenses, work by gently reshaping the front surface of your eye while you sleep so you can see clearly without glasses or daytime contacts. They’re rigid, gas-permeable lenses with a precisely engineered curve that flattens the central cornea just enough to correct your focus. You put them in at bedtime, take them out in the morning, and go about your day with improved vision.
How the Reshaping Actually Happens
The key force at work isn’t the lens pressing directly on your eye like a mold. Instead, it’s the thin layer of tears trapped between the lens and your cornea that does the work. When you close your eyes and the lens settles into position, this tear film gets compressed unevenly. The lens is designed so tears are thinnest at the center and thicker around the edges, creating a pattern of fluid pressure that pushes the outermost corneal cells (the epithelium) from the center toward the periphery.
This redistribution of cells flattens the central cornea by tiny but optically meaningful amounts. The deeper layers of the cornea aren’t significantly affected. The change is entirely in the epithelium, which is only about five or six cell layers thick. That’s also why the effect is temporary: those surface cells naturally migrate back to their original positions once you stop wearing the lenses.
How Quickly You’ll Notice Results
Most people notice a difference after a single night, though vision won’t be sharp yet. By days three to five of consistent nightly wear, many reach nearly full correction. The sweet spot for most patients is 10 to 14 days, at which point vision typically stabilizes at its best level. People with milder prescriptions tend to reach full correction faster, while higher prescriptions take longer to settle in.
During this break-in period, your eye doctor may give you temporary soft lenses or updated glasses to bridge the gap on days when your correction isn’t quite there yet.
What They Can and Can’t Correct
Ortho-k works best for mild to moderate nearsightedness. The FDA approved one major lens design for up to negative 6 diopters of myopia, but most lens manufacturers set their practical treatment range between negative 3.50 and negative 4.50 diopters. Astigmatism correction is more limited, typically capped at 1.50 diopters for the most common type and 0.75 diopters for less common orientations.
If your prescription falls outside these ranges, the lenses simply can’t redistribute enough corneal tissue to fully correct your vision. You might still get a partial benefit, but you’d likely need glasses for some activities.
Why They’re Safe to Sleep In
Sleeping in contact lenses is normally risky because closed eyelids already reduce oxygen supply to the cornea, and a standard lens on top of that can starve the tissue. Ortho-k lenses are made from highly oxygen-permeable rigid materials. The critical measurement is called Dk/t (oxygen transmissibility relative to lens thickness), and research has established that a value of 87 or higher is needed for safe overnight wear. Modern ortho-k lenses meet or exceed this threshold, allowing enough oxygen through to keep the cornea healthy during sleep.
Infection Risk
The most serious risk with any overnight lens wear is microbial keratitis, a corneal infection that can threaten vision if not treated promptly. A large multicenter study found an incidence of 5.4 cases per 10,000 patient-years among ortho-k wearers. That’s low in absolute terms, but not zero, and almost all cases trace back to poor lens hygiene.
Proper cleaning makes the difference. You can use either a multipurpose solution designed for gas-permeable lenses or a hydrogen peroxide-based system. Hydrogen peroxide systems are preservative-free, which is helpful if you’re sensitive to the chemicals in multipurpose solutions, but they require a neutralization step before you can put the lenses back in your eyes. Whichever system you use, the basics matter: wash your hands before handling lenses, rub and rinse lenses after each use, replace your lens case regularly, and never use tap water.
What Happens When You Stop
The reshaping effect is fully reversible. If you stop wearing the lenses, your cornea gradually returns to its original shape. Within about two weeks, measurable changes in corneal curvature are already underway. By one month after stopping, studies show that corneal shape returns to baseline values. This is a feature for some people (no permanent commitment) and a limitation for others (you have to wear them every night, or at least most nights, to maintain clear daytime vision).
Slowing Nearsightedness in Children
Beyond correcting vision day to day, ortho-k has a second use that’s driven much of its popularity: slowing the progression of myopia in children. As kids grow, their eyes can elongate, making nearsightedness steadily worse. In a study that tracked children wearing an ortho-k lens in one eye and a regular lens in the other, the control eyes grew by 0.36 mm in axial length over 12 months, while the ortho-k eyes grew by only 0.07 mm. Over two years, the ortho-k eyes showed no statistically significant elongation at all, while the untreated eyes continued to grow.
This matters because higher myopia later in life carries increased risk of retinal problems, glaucoma, and other complications. Slowing that progression during childhood, when the eye is growing fastest, can make a meaningful difference in long-term eye health. Ortho-k is typically considered for children as young as five or six, though the fitting process requires a cooperative child who can handle rigid lens insertion and removal with parental help.
The Fitting Process
Getting ortho-k lenses isn’t as simple as picking up a box at the pharmacy. Your eye doctor maps the surface of your cornea in detail using a topographer, which creates a 3D elevation map of the eye. The lenses are then custom-designed based on that map, with the curvature calculated to create the exact tear film pressure pattern needed for your prescription. You’ll have several follow-up visits in the first few weeks to check how the cornea is responding and fine-tune the fit if needed.
People with certain eye conditions, including active infections, severe dry eye, or irregular corneal shapes like keratoconus, aren’t good candidates. A healthy, normally shaped cornea is essential for the lenses to work predictably and safely.