Orthokeratology (or Ortho K) involves the gentle reshaping of the cornea in order to eliminate the need for glasses or regular contact lenses. Ortho K uses hard contact lenses that are worn overnight, which alter the shape of the cornea enough to provide the wearer with clear vision throughout the day. As the reshaping of the cornea is temporary, the lenses must be worn every night otherwise the effect is reversible. Initially it may take a few nights wear before good vision is achieved.