The material commonly known as “sleep crust” or “sleep sand” is experienced by nearly everyone. Medically, this substance is often referred to as rheum or gound, representing a natural byproduct of the eye’s continuous cleaning processes. This accumulation is harmless and indicates that your eyes are functioning normally throughout the sleep cycle.
What Sleep Crust Is Made Of
The eye maintains a constant protective layer known as the tear film, composed of three distinct layers produced by specialized glands. This film includes a watery layer from the lacrimal glands, an oily layer from the meibomian glands, and a mucus layer secreted by goblet cells on the conjunctiva. The oily layer prevents rapid evaporation of the water, while the mucus layer helps the film adhere evenly to the eye’s surface.
Rheum is this complex tear film material that has dried up and mixed with various other substances. Throughout the day, the tear film traps environmental particulates like dust, pollen, and foreign debris from the air. It also collects old, sloughed-off epithelial cells from the surface of the eye.
When these components mix and dry, they form the sticky or gritty substance found upon waking, often accumulating near the inner corner of the eye. The consistency of the sleep crust, whether soft and moist or hard and dry, depends on the ratio of the watery and oily components present. This accumulation is a direct result of the eye’s defense mechanism, trapping irritants before they can cause harm.
The Mechanism of Nighttime Accumulation
The process of blinking is the primary reason this material does not accumulate during waking hours. Blinking acts as a biological windshield wiper, constantly replenishing the tear film and sweeping debris toward the tear ducts for drainage. This action efficiently flushes away developing rheum before it can dry out and solidify.
When a person sleeps, blinking ceases completely. The protective material the eye continues to produce is no longer actively pumped away from the surface. This allows accumulated debris and tear components to settle undisturbed in the corners of the eye, particularly the inner canthus.
Tear production is naturally reduced during sleep, and the tear ducts operate less efficiently than when we are awake. With less fluid movement and drainage, the water content of the collected debris evaporates slowly over several hours of rest. This evaporation leaves behind the solid, dried mixture of mucus, oil, and trapped particulates that constitutes the sleep crust.
Signs That Eye Discharge Is Not Normal
While some morning rheum is normal, a sudden change in the appearance or amount of eye discharge can indicate a problem that requires medical attention. Normal sleep crust is usually clear, whitish, or light gray, and either dry and flaky or slightly moist. Discharge that is dramatically different in color is a sign of concern, such as a thick, continuous flow that is yellow, green, or gray, potentially indicating a bacterial infection.
A significant, sudden increase in stickiness or volume is also a red flag, as this excessive discharge may be symptomatic of an infection like bacterial or viral conjunctivitis, commonly known as pink eye. In bacterial infections, the discharge can be so profuse and pus-filled that it effectively glues the eyelids shut, making it difficult or impossible to open the eyes upon waking. This excessive stickiness is a sign that the body is producing large amounts of inflammatory material to fight a localized infection.
Other symptoms that accompany abnormal discharge should not be ignored, including intense eye pain, heightened redness of the whites of the eyes or the eyelids, and increased sensitivity to light. If the discharge is constantly watery and stringy rather than crusty, it might indicate allergic conjunctivitis or a foreign body irritation, suggesting a different cause than a typical bacterial issue. Any persistent or painful discharge that lasts more than a few days warrants a professional examination by an eye care specialist.
If the crust is excessive or sticky, it should be removed gently using a clean, warm compress. Apply the compress to the closed eye for a few minutes to soften the material before wiping it away, always wiping from the inner corner outward. It is important to always use a clean cloth for each eye and to wash hands thoroughly before and after touching the eyes to prevent the spread of potential infections.