before sleeping. An "extended wear" (EW) contact lens is designed for continuous overnight wear, typically for up to 6 consecutive nights. Newer materials, such as silicone hydrogels, allow for even longer wear periods of up to 30 consecutive nights; these longer-wear lenses are often referred to as "continuous wear" (CW). EW and CW contact lenses can be worn overnight because of their high oxygen permeability. While awake, the eyes are mostly open, allowing oxygen from the air to dissolve into the tears and pass through the lens to the cornea. While asleep, oxygen is supplied from the blood vessels in the back of the eyelid. A lens hindering passage of oxygen to the cornea causes corneal hypoxia which can result in serious complications, such as corneal ulcer that, if left untreated, can permanently decrease vision. EW and CW contact lenses typically allow for a transfer of 5–6 times more oxygen than conventional softs, allowing the cornea to remain healthy, even with closed eyelids. Wearing lenses designed for daily wear overnight has an increased risk for corneal infections, corneal ulcers and corneal neovascularization—this latter condition, once it sets in, cannot be reversed and will eventually spoil vision acuity through diminishing corneal transparency. The most common complication of extended wear is giant papillary conjunctivitis (GPC), sometimes associated with a poorly fitting contact lens. Replacement schedule Contact lenses are often categorized by their replacement schedule. Single use lenses (called 1-day or daily disposables) are discarded after one use. Because they do not have to stand up to the wear and tear of repeated uses, these lenses can be made thinner and lighter, greatly improving their comfort. Lenses replaced frequently gather fewer deposits of allergens and germs, making these lenses preferable for patients with ocular allergies or for those who are prone to infection. Single-use lenses are also useful for people who wear contact lenses infrequently, or when losing a lens is likely or not easily replaced (such as when on vacation). They are also considered useful for children because cleaning or disinfecting is not needed, leading to improved compliance. Other disposable contact lenses are designed for replacement every two or four weeks. Quarterly or annual lenses, which used to be very common, are now much less so. Rigid gas permeable lenses are very durable and may last for several years without the need for replacement. PMMA hards were very durable and were commonly