Eyes turn red when tiny blood vessels on the surface of the eye widen and fill with more blood than usual. This dilation can be triggered by dozens of different causes, from a night of poor sleep to a serious infection. Most of the time, red eyes are harmless and temporary, but certain patterns of redness signal something that needs prompt attention.
What Happens Inside the Eye
The white part of your eye is covered by a thin, transparent membrane called the conjunctiva, which contains a dense network of microscopic blood vessels. Normally these vessels are so small you can barely see them. When something irritates or inflames the eye, your body releases chemical signals that cause those vessels to relax and expand. More blood flows through, and the white of your eye takes on a pink or red appearance.
Three main pathways drive this response. The most familiar is the histamine pathway: when your immune system detects an allergen or irritant, it releases histamine, which acts on the vessel walls and causes them to relax. Inflammatory signals from the immune system can also trigger redness through a separate set of chemical messengers. And sensory nerves in the eye itself can release compounds that dilate blood vessels directly, which is why even a speck of dust can make your eye visibly red within seconds.
Allergies, Infections, and Pink Eye
Conjunctivitis, commonly called pink eye, is one of the most frequent causes of red eyes. The type of discharge and the level of itchiness can help you tell the three main forms apart.
- Allergic conjunctivitis causes intense itching and a watery or stringy discharge. It typically affects both eyes and flares up around pollen, dust, or pet dander. This type is not contagious.
- Viral conjunctivitis produces minimal itching and a watery discharge, often starting in one eye before spreading to the other. Adenovirus, the most common culprit, is highly contagious. Transmission risk ranges from 10 to 50%, and the virus can survive on surfaces for weeks.
- Bacterial conjunctivitis is characterized by thick, yellowish or greenish discharge that may crust your eyelids shut overnight. Itching is usually minimal compared to allergic cases.
The contagious forms spread easily through direct contact and shared items like towels and pillowcases. If one eye turns red with discharge, keeping your hands away from your face and washing them frequently can help prevent spreading it to the other eye or to other people.
Dry Eyes and Screen Time
Dry eye disease is a leading cause of chronic, low-grade redness. When the tear film that coats your eye becomes unstable, whether from reduced tear production or excessive evaporation, the exposed surface cells become stressed. That stress activates immune cells on the surface, which release inflammatory signals and trigger a self-reinforcing cycle: inflammation damages the surface, the damage makes tears less stable, and instability drives more inflammation.
Screen use makes this significantly worse. You normally blink about 14 to 16 times per minute, but during concentrated screen work that rate can drop to as few as 4 to 6 blinks per minute. Some studies have recorded rates as low as 3.6 blinks per minute during computer use. Each blink spreads a fresh layer of tears across the eye, so fewer blinks means more evaporation and more surface dryness. Digital eye strain affected roughly 50 to 60% of children during the COVID-19 era of increased screen time, and one large survey found that 78% of participants reported at least one symptom related to screen-induced eye strain.
Eyelid Inflammation
Blepharitis, or inflammation along the eyelid margin, is one of the most common eye conditions and a frequently overlooked source of persistent redness. The posterior form involves the meibomian glands, tiny oil-producing glands that line the inner edge of your eyelids. These glands secrete an oily layer that sits on top of your tears and slows evaporation. When they become blocked or dysfunctional, tears evaporate too quickly, leaving the corneal surface vulnerable. The result is redness, a gritty foreign-body sensation, light sensitivity, and sometimes blurred vision. People with rosacea, eczema, or other skin conditions are more prone to this type of eyelid inflammation.
Contact Lens Overwear
Contact lenses sit directly on the cornea and reduce the amount of oxygen that reaches it. Short-term, this can cause mild swelling of the corneal surface, which is usually reversible. Over months or years of excessive wear, the oxygen deprivation becomes more damaging. The cornea may respond by growing new blood vessels into tissue that is normally vessel-free, a process called neovascularization. This can be superficial or deep and, in severe cases, can involve the full circumference of the cornea. Prolonged lens wear can also permanently change the shape of the cornea, inducing astigmatism, and damage the innermost cell layer in ways that complicate future eye surgeries.
If your eyes are consistently red after wearing contacts, especially if the redness worsens toward the end of the day, it may be a sign that your lenses are limiting oxygen delivery. Switching to higher-oxygen lens materials, reducing daily wear time, or alternating with glasses can help.
Broken Blood Vessels
A subconjunctival hemorrhage looks alarming: a bright red patch spreads across part of the white of your eye, sometimes covering a large area. Despite its appearance, it is usually painless, does not affect vision, and resolves on its own. A small vessel simply bursts under the conjunctiva, often from coughing, sneezing, straining, or rubbing your eyes. The blood typically clears within 7 to 14 days, though larger bleeds can take up to 21 days to fully reabsorb. No treatment is needed in most cases.
Environmental and Lifestyle Triggers
A number of everyday exposures can cause temporary redness without any underlying disease. Chlorinated pool water strips away the protective tear film. Smoke, air pollution, and wind dry and irritate the surface. Cosmetics and skincare products that migrate into the eye can trigger both irritation and allergic reactions. Alcohol dilates blood vessels throughout the body, including in the eyes, and poor sleep reduces tear production overnight, leaving eyes dry and red in the morning.
When Red Eyes Signal an Emergency
Most red eyes are not dangerous, but a few combinations of symptoms point to conditions that can threaten your vision. Acute angle-closure glaucoma causes sudden, severe pain in one eye along with blurred vision, halos around lights, and sometimes nausea or vomiting. The pupil on the affected side is typically fixed and mid-dilated, meaning it does not respond normally to light. This is an emergency that can cause rapid, permanent vision loss without treatment.
Other warning signs that warrant immediate evaluation:
- Sudden vision changes alongside redness
- Severe eye pain or headache with a red eye
- Sensitivity to light that was not previously present
- Halos around lights appearing for the first time
- Swelling in or around the eye that makes it hard to open
- Chemical splash or foreign object in the eye
- Nausea or vomiting paired with eye pain
Red eyes that are painless, mildly irritated, and not affecting your vision are rarely urgent. But redness that comes on suddenly with pain, vision changes, or light sensitivity is a different category entirely and should be evaluated the same day.