Halos around lights are caused by light scattering or diffracting as it passes through your eye’s cornea and lens. In many cases, the cause is temporary and harmless, like dry eyes or smudged contact lenses. But halos can also signal conditions that need treatment, including cataracts, corneal swelling, and glaucoma.
How Your Eye Creates Halos
To reach your retina, light passes through two transparent structures: the cornea (the clear front window) and the lens (sitting just behind the pupil). Both contain microscopic fibers and membranes that can scatter light slightly off course. When that scattering increases for any reason, you see a soft ring of light radiating outward from a bright source, especially at night when your pupil is wide open and letting in more peripheral light.
The cornea’s transparency comes from tightly packed collagen fibrils arranged in a near-perfect lattice, transmitting more than 95% of incoming light directly. When that arrangement is disrupted by swelling, dryness, or disease, light scatters in all directions and creates a diffuse glow. The lens works differently. Its fiber cell membranes scatter light in a more focused, forward pattern with a spread of about 3 degrees, producing the distinct circular halo you notice around headlights or streetlamps.
Dry Eyes and Tear Film Problems
Your tear film is technically the first lens light passes through before reaching the cornea. When it’s smooth and even, light enters cleanly. When it breaks up or thins out, the corneal surface becomes temporarily uneven, scattering light into halos, starbursts, or smeared streaks. This is one of the most common reasons people notice halos while driving at night, and it tends to be worse after long stretches of screen time or in dry, air-conditioned environments. Blinking a few times may briefly improve the halo because it redistributes the tear film across the surface of the eye.
Cataracts
As the lens ages and develops cloudy areas, it scatters significantly more light. Cataracts located near the back center of the lens are particularly disruptive because they sit right in the path of focused light heading toward the retina. Even a small opacity in that position can cause noticeable glare and halos, sometimes before standard vision tests pick up any problem. Cataracts develop slowly, so halos from this cause tend to creep in over months or years rather than appearing suddenly.
Corneal Swelling
The cornea stays clear partly because a layer of pump cells on its inner surface constantly removes excess fluid. When those cells are damaged or overwhelmed, the cornea absorbs water and swells, disrupting the precise collagen arrangement that normally keeps it transparent.
A condition called Fuchs’ dystrophy is a classic example. The pump cells gradually die off, and the cornea retains fluid. In early stages, vision is blurry and hazy in the morning (fluid accumulates overnight) but clears during the day as the cornea dries out. As the condition progresses, halos around lights and poor night vision become persistent symptoms.
Contact lens wear can cause a milder version of the same problem. The cornea has no blood supply and gets its oxygen directly from the air and from tears. A contact lens reduces that oxygen delivery, which can cause mild corneal swelling. This is more likely with extended wear, sleeping in lenses, or using lenses past their replacement date.
Glaucoma
Halos that appear suddenly alongside eye pain, headache, nausea, or vomiting can signal acute angle-closure glaucoma, a condition where fluid drainage inside the eye is abruptly blocked and pressure spikes. The rapid pressure increase forces fluid into the cornea, causing it to swell and scatter light into rainbow-colored rings around lights. This is an eye emergency. Without treatment within hours, permanent vision loss can occur.
Chronic forms of glaucoma typically don’t cause halos on their own, which is why the sudden onset combined with pain is the key distinction.
After LASIK or Other Eye Surgery
Halos are a well-known side effect after LASIK and similar refractive procedures. The surgery reshapes the cornea, and during healing, the reshaped surface can scatter light differently than before. Glare and halos tend to be most noticeable in the first few days to weeks after surgery and gradually resolve as the cornea heals and stabilizes. For most people, these symptoms fade within a few weeks, though a small number of patients experience them for longer.
Medications
Certain medications can alter the way your eyes process light. Drugs used for autoimmune conditions, overactive bladder, erectile dysfunction, and some cancer treatments have all been documented to cause halo vision as a side effect. If halos appear shortly after starting a new medication, that timing is worth mentioning to your prescriber.
How Eye Doctors Identify the Cause
An eye care provider can usually pinpoint the source of halos during a standard exam. The primary tool is a slit lamp, a specialized microscope with an adjustable beam of light that lets the examiner see through each layer of the eye individually. By changing the angle and thickness of the light, they can spot corneal swelling, lens cloudiness, dry patches on the surface, and signs of elevated eye pressure. If corneal damage is suspected, a harmless dye dropped onto the eye highlights scratches or irregularities under the slit lamp’s light.
Pressure inside the eye is measured separately, usually with a quick, painless test that either uses a puff of air or a small probe that briefly touches the numbed corneal surface. Together, these tests distinguish between corneal causes (dry eye, swelling, surface damage), lens causes (cataracts), and pressure-related causes (glaucoma).
When Halos Need Urgent Attention
Occasional halos around bright lights at night, especially when your eyes are tired or dry, are common and rarely serious. The pattern that demands immediate attention is halos appearing suddenly alongside other symptoms: eye pain, headache, nausea, vomiting, or a noticeable drop in vision. That combination suggests acute angle-closure glaucoma or another condition where pressure inside the eye is rising fast. Halos that are new, persistent, or worsening over weeks also warrant an eye exam, even without pain, because they can be the first sign of cataracts or corneal disease before other symptoms develop.