Lenses designed for overnight or continuous wear are usually made from a material – silicone hydrogel – designed to allow more oxygen through to the eye than conventional soft lens materials. Oxygen is essential for the health of the eye. Although conventional materials are fine for use during the day - when the eye is open, they do not let enough oxygen through when the lids are closed for a long period, overnight, putting the tissues of the eye under stress. During the 1970s and 80s, previous lenses suitable for extended wear were made from materials which were less permeable to oxygen. A number of people who tried extended wear at that time had complications such as irritation and even infection. Current generation lenses are safe and comfortable when wearers follow the care programme of a qualified optician. As with all lenses, good hygiene is essential to avoid the possibility of infection and if wearers experience any problems, they should consult their optician urgently.