Orthokeratology (ortho-k) is the fitting of specially designed gas permeable contact lenses (RGP) that you wear overnight. While you are asleep, the lenses gently reshape the front surface of your eye (cornea) so you can see clearly the following day after you remove the lenses when you wake up. Ortho-k lenses are prescribed for two purposes: To correct refractive errors To slow the progression of childhood myopia