Sleeping in contact lenses is strongly discouraged by eye care professionals. Lenses are medical devices, and wearing them beyond the prescribed schedule significantly elevates the risk of serious complications. Understanding why this prohibition exists requires examining the physiological changes that occur when a lens remains on the eye during sleep. This article explains the underlying biological mechanisms that turn an overnight choice into a potential threat to vision health.
How Contact Lenses Restrict Corneal Oxygen Supply
The cornea lacks blood vessels to maintain its transparency, receiving oxygen primarily from the atmosphere when the eye is open. When the eye is closed during sleep, the oxygen supply drops significantly, as the cornea must rely on the blood vessels in the inner eyelid. A contact lens acts as a physical barrier, severely restricting this reduced oxygen flow, leading to corneal hypoxia. This oxygen deprivation prevents the cornea from maintaining proper fluid balance. As a result, the corneal tissue swells, a condition known as corneal edema. This swelling causes vision to become temporarily blurred and compromises the cornea’s structural integrity, making it more susceptible to injury and infection.
The Compromise of Natural Eye Defenses
Beyond hypoxia, sleeping in lenses sabotages the eye’s natural defense mechanisms. The tear film performs the functions of lubrication, nutrient delivery, and waste removal. It also contains natural antimicrobial agents, such as lysozyme, which constantly fight off pathogens. When a lens is placed on the eye, it divides the tear film into two thinner layers, disrupting its protective stability. Blinking usually serves to flush debris, dead cells, and trapped bacteria from the surface of the eye. During sleep, this flushing action stops entirely, allowing contaminants to stagnate and accumulate underneath the lens. This trapping effect concentrates harmful microorganisms against the compromised corneal surface, while the natural antimicrobial agents are not effectively circulated or replenished.
Severe Risks of Corneal Infection and Damage
The combination of a structurally weakened, oxygen-deprived cornea and trapped microorganisms leads directly to the primary danger: microbial keratitis. This severe infection of the cornea can quickly progress to a corneal ulcer. Sleeping in contact lenses significantly increases the risk of this sight-threatening condition. Infections are often caused by common bacteria, such as Pseudomonas aeruginosa, which rapidly destroy corneal tissue. Devastating infections are also caused by organisms like Acanthamoeba, which are often linked to water exposure and are difficult to treat. These infections can result in permanent corneal scarring, which blocks light from entering the eye, leading to irreversible vision loss or blindness. Chronic hypoxia can also induce corneal neovascularization, causing new blood vessels to grow into the normally avascular cornea in an attempt to supply oxygen, a process that permanently impairs clarity.
Understanding Extended Wear Options
To address hypoxia risks, contact lens technology includes extended wear options, often made from highly oxygen-permeable silicone hydrogel materials. These specialized lenses allow a greater flow of oxygen to the cornea, even when the eye is closed, and some are approved for continuous wear for up to 30 days. While these lenses largely mitigate the risk of corneal hypoxia, they do not eliminate the danger of infection. The higher risk of microbial keratitis associated with overnight wear is linked to the wearing modality itself—the trapping of debris and the cessation of tear flow—rather than just the lens material. Therefore, wearing them overnight still increases the risk of infection compared to daily wear, and it should only be done with strict adherence to a schedule prescribed and monitored by an eye care professional.