Eyes turn red when tiny blood vessels on the surface of the eye widen and fill with more blood than usual. This process, called vasodilation, is the eye’s basic response to irritation, infection, dryness, or injury. The triggers range from staring at a screen too long to a serious infection, but the underlying mechanism is almost always the same: something provokes inflammation, and the blood vessels expand to deliver immune cells to the area.
The inflammation is driven by molecules like histamine and cytokines, which signal blood vessels in the conjunctiva (the thin, clear tissue covering the white of the eye) to open up. That rush of blood is what gives the eye its pink or red appearance.
Infections: Pink Eye and Its Variations
Conjunctivitis, commonly called pink eye, is one of the most frequent causes of red eyes, and the type of infection changes what you’ll experience. Bacterial conjunctivitis produces moderate redness along with a thick yellow or green discharge that can crust over your eyelashes, especially overnight. It often looks worse than it feels.
Viral conjunctivitis tends to be more painful. The redness is moderate, but you’ll likely feel a sandy, gritty sensation as if something is stuck in your eye, and light may become uncomfortably bright. The discharge, if any, is usually watery rather than thick.
Allergic conjunctivitis looks different from both. The redness is typically mild, the discharge is clear and watery, and itching is the dominant symptom. Pollen, pet dander, and dust mites are the usual culprits. If your red eyes come with intense itching and no pain or crusty discharge, allergies are the most likely explanation.
Dry Eyes and Tear Film Problems
Your eyes depend on a thin, stable layer of tears to stay comfortable and clear. When that layer breaks down, the surface dries out and becomes inflamed, turning the eyes red. One common cause is dysfunction of the meibomian glands, small oil-producing glands along your eyelid margins. When these glands become blocked or produce poor-quality oil, the tear film evaporates too quickly. Over time, this leads to chronic irritation, redness, and a gritty feeling that doesn’t go away on its own.
Screen use makes things considerably worse. People normally blink about 15 times per minute, but during computer or phone use, that drops to just 5 to 7 blinks per minute. Since blinking is what spreads moisture across the eye’s surface, cutting your blink rate by more than half leaves the surface exposed and prone to drying out. If your eyes are consistently red by the end of a workday, this reduced blink rate is a likely factor. Making a conscious effort to blink more often, and following the 20-20-20 rule (every 20 minutes, look at something 20 feet away for 20 seconds), can help.
Broken Blood Vessels
Sometimes redness appears not as a diffuse pinkness but as a bright red patch on the white of the eye. This is a subconjunctival hemorrhage, a small blood vessel that has burst and leaked blood under the surface. It looks alarming but is almost always harmless.
Common triggers include coughing, sneezing, vomiting, straining on the toilet, heavy lifting, or rubbing your eyes too hard. Anything that briefly spikes pressure in the veins near your eye can cause it. Contact lens wear and minor eye injuries are also common causes. Most of these spots heal on their own within two weeks, though larger ones can take longer. No treatment is needed.
Environmental and Lifestyle Triggers
Beyond screens, a number of everyday environmental factors can push your eyes toward redness. Chlorine in swimming pools strips away the protective tear film. Smoke, dust, and strong winds expose the eye’s surface to direct irritation. Air conditioning and forced-air heating reduce ambient moisture, which accelerates tear evaporation. Even indoor air quality plays a role: environments with high levels of particulate matter or microbial growth can provoke low-grade inflammation on the eye’s surface.
Alcohol is another common trigger. It dilates blood vessels throughout the body, including those in the conjunctiva, producing temporary redness that resolves as the alcohol is metabolized. Sleep deprivation has a similar vasodilatory effect.
Why “Get the Red Out” Drops Can Backfire
Over-the-counter redness-relief eye drops work by forcing the dilated blood vessels to constrict. They produce rapid results, which is why people reach for them. But these drops offer no long-term benefit. With regular use, the blood vessels begin to lose their responsiveness to the active ingredient, and when the drops wear off, the vessels rebound to an even wider diameter than before. This rebound redness creates a cycle where you need the drops more frequently just to look normal.
Artificial tears (lubricating drops without a redness-reducing agent) are a safer choice for everyday irritation. They address dryness, one of the most common root causes of redness, without triggering rebound effects.
When Red Eyes Signal Something Serious
Most red eyes are caused by something minor and self-limiting. But certain accompanying symptoms point to conditions that need prompt attention from an eye specialist. These include:
- Eye pain that goes beyond mild irritation or grittiness
- Sensitivity to light that makes it hard to keep the eye open
- Changes in vision, including blurriness, double vision, or partial loss of your visual field
- Flashes of light or a sudden onset of floaters
- A significant eye injury or blow to the head or eye area
- A visible opacity or cloudiness over the normally clear parts of the eye
These symptoms can indicate conditions like acute glaucoma, uveitis (inflammation inside the eye), or a corneal ulcer, all of which can threaten vision if not treated quickly. Redness combined with severe pain and light sensitivity is a particularly important combination to take seriously. Redness on its own, without pain or vision changes, is rarely an emergency.