OKL are custom fit, form stable lenses used during sleep. The lenses induce a temporal flattening of the cornea, which modify or eliminate refractive errors, leaving no need for glasses or contact lenses during daytime. OKL were introduced in the 1960 ́s, but with limited success due the lens material (PMMA) which was not gas-permeable. Decentration was also a problem making the effect variable and unpredictable. With new gas- permeable lens materials and computer assisted topographic measurements of the cornea, fitting of the lenses has been improved significantly, so that nightly wear is now standard treatment.