The acronym D/N, frequently seen on contact lens packaging, stands for Day/Night. This designation indicates that the lenses are approved by regulatory bodies, such as the U.S. Food and Drug Administration (FDA), for extended wear, meaning they can be worn while sleeping. D/N lenses are designed to be worn for a period longer than a single day without removal, differentiating them from standard daily-wear contacts. The ability to wear a lens overnight is a specialized approval based on the lens material and design. A D/N prescription requires the oversight of an eye care professional, who determines the maximum safe wearing schedule for the individual patient.
Understanding Extended and Continuous Wear
The Day/Night classification covers two categories of overnight contact lens use: Extended Wear (EW) and Continuous Wear (CW). Extended Wear typically refers to a schedule where lenses are worn for a specified number of days and nights, often up to six nights and seven days, before being removed for cleaning and disinfection. This practice offers convenience while incorporating a weekly cleaning break.
Continuous Wear allows the wearer to keep the lenses in for up to 30 consecutive days and nights. The specific schedule is determined by the lens manufacturer’s approval and the eye doctor’s professional judgment regarding the patient’s eye health. The D/N label is an umbrella term, and the specific wear period must always follow the individual prescription.
The Role of Oxygen Permeability in D/N Lenses
The cornea receives its oxygen directly from the atmosphere, not from blood vessels. A contact lens acts as a barrier when the eye is open, and the oxygen supply is further reduced when the eye is closed during sleep. Traditional soft lenses restricted oxygen flow significantly, making overnight wear dangerous and leading to corneal hypoxia.
The development of silicone hydrogel materials made D/N lenses possible. This material incorporates silicone, allowing for a much higher rate of oxygen transmission to the cornea compared to older hydrogel lenses. Oxygen transmissibility is quantified by the Dk/t value, which measures the lens material’s oxygen permeability (Dk) relative to its thickness (t). For a contact lens to be considered safe for overnight wear, it requires a Dk/t value of 125 or greater to ensure the cornea receives enough oxygen.
Health Risks Associated with Overnight Contact Use
Even with highly oxygen-permeable D/N lenses, sleeping in contacts carries an elevated risk of serious eye health issues. The primary concern is microbial keratitis, a potentially blinding infection of the cornea. Wearing lenses overnight increases the risk of this infection by six- to eightfold.
When a contact lens is worn overnight, it traps microorganisms between the lens and the eye’s surface, creating a warm, moist environment where bacteria and fungi can thrive. This increased risk is due to reduced oxygen, which compromises the cornea’s defenses, and the accumulation of deposits on the lens surface. Another risk is corneal edema, the swelling of the cornea due to insufficient oxygen supply, leading to blurred vision and discomfort.
A complication that can arise from extended wear is Giant Papillary Conjunctivitis (GPC), an inflammatory reaction where small bumps form on the inner surface of the eyelid. GPC is caused by deposits that build up on the lens surface, leading to chronic irritation and discomfort that necessitates discontinuing lens wear. Because of these risks, D/N lenses must be used with caution, requiring strict adherence to the wearing schedule and frequent check-ups with an eye care professional to monitor corneal health.