Extended wear lenses, often referred to as “leave-in” contacts, are approved for wear during sleep, offering seamless vision correction upon waking. Their primary attraction is the convenience of eliminating the daily routine of lens removal, cleaning, and reinsertion, which appeals to people with unpredictable schedules or those who value constant vision correction. While sleeping in lenses is a significant technological advance, this convenience introduces unique physiological challenges for the eye. Understanding the material science and the specific health risks involved is necessary for anyone considering this type of lens wear.
The Material Science Behind Extended Wear Lenses
The fundamental difference between contacts approved for overnight wear and traditional lenses is their ability to transmit oxygen to the cornea. The cornea lacks blood vessels and receives oxygen from the air, but when the eyelid is closed during sleep, the oxygen supply is significantly reduced. Conventional lenses, made of standard hydrogel material, restrict oxygen flow during this closed-eye state, leading to corneal hypoxia.
To address this, manufacturers developed silicone hydrogel materials, which transport oxygen far more efficiently. Oxygen transmissibility (Dk/t value) quantifies how much oxygen reaches the cornea through the lens. Silicone hydrogel lenses have a substantially higher Dk/t value, allowing them to meet the cornea’s oxygen demands even when the eyes are closed, helping prevent complications like corneal swelling.
Adherence to Prescribed Wearing Schedules
The term “leave-in” does not mean these lenses can be worn indefinitely; their use is governed by specific regulatory and manufacturer guidelines. Extended wear (EW) refers to lenses approved for continuous overnight use for up to six nights and seven days, requiring removal for at least one full night afterward. Continuous wear (CW) is a subset approved for up to 30 consecutive nights of wear.
The final, specific wearing schedule must be prescribed by an eye care professional. The provider evaluates individual factors, such as tear film quality and corneal health, to determine the maximum safe duration for the patient. Strict compliance with this guidance is necessary, as wearing lenses beyond the recommended period significantly increases the risk of complications due to the accumulation of deposits and debris.
Understanding the Specific Health Risks
Despite material advancements, sleeping in contact lenses carries a significantly higher risk of serious eye health complications compared to daily wear. The most severe concern is microbial keratitis (MK), a sight-threatening corneal infection that can lead to permanent vision loss if not treated promptly. Studies consistently show that the risk of developing MK is elevated when lenses are worn overnight.
The increased risk during sleep stems from several factors. Tear stagnation under the closed eyelid reduces the natural flushing of debris and microbes. The lens provides a surface for pathogens, such as Pseudomonas aeruginosa, to adhere to and form biofilms. Furthermore, the reduced oxygen environment, even with high Dk/t lenses, can weaken the cornea’s protective epithelial layer, making it more susceptible to microbial invasion.
Contact Lens-induced Acute Red Eye (CLARE)
Another complication is Contact Lens-induced Acute Red Eye (CLARE), an inflammatory reaction that typically occurs upon waking after sleeping in lenses. CLARE is associated with the buildup of bacterial toxins on the lens surface and is characterized by a painful, very red eye. While high oxygen transmissibility has reduced complications related to oxygen deprivation, it has not eliminated the fundamental risk of infection and inflammation associated with continuous lens wear.