The experience of seeing halos, streaks, or excessive glare around light sources after the sun goes down is a common visual phenomenon. This light distortion, which can manifest as starbursts radiating from headlights or a hazy ring around streetlamps, happens because the eye is unable to perfectly focus a concentrated point of light in low-light conditions. While this blurring affects nearly everyone, for some people, it signals a change in the physical structure of the eye that requires professional attention. Understanding the mechanics of night vision helps clarify why this distortion occurs.
Why Vision Changes in Low Light
The primary physiological reason for lights appearing blurry at night relates to the size of the pupil, the eye’s aperture. In darkness, the pupil automatically widens (mydriasis) to allow the maximum amount of light to reach the retina. This necessary dilation exposes the peripheral edges of the cornea and lens, which are less optically perfect than the center.
Light rays passing through these outer edges bend differently than those going through the center, a phenomenon known as spherical aberration. This imperfection prevents light from focusing on a single point on the retina, causing the image to appear blurred and streaked. This increase in aberration often results in a slight shift toward nearsightedness, or myopia, sometimes referred to as “night myopia.”
How Refractive Errors Affect Night Vision
While spherical aberration is a universal issue, existing refractive errors significantly worsen light distortion. The most common structural issue is astigmatism, which occurs when the cornea or the lens is shaped more like a football than a perfect sphere. This irregular curvature causes light entering the eye to focus unevenly, splitting the light rays instead of converging them to a sharp point.
During the day, the small pupil size may block the most distorted peripheral light, allowing the brain to compensate. However, when the pupil dilates at night, it exposes the full extent of the uneven surface, magnifying the light-splitting effect. This structural issue is the typical cause for the pronounced starbursts or streaks radiating from point light sources, and uncorrected nearsightedness (myopia) also contributes to generalized blurriness in low light conditions.
Medical Conditions That Intensify Blurriness
Beyond common refractive errors, certain acquired medical conditions can severely intensify night glare and halos. A common culprit is the development of cataracts, which is a clouding of the eye’s naturally clear lens. The clouded lens scatters incoming light rather than transmitting it cleanly.
This light scattering creates noticeable halos and significant glare around lights at night, often serving as one of the first recognized symptoms of a cataract.
Other Contributing Conditions
An unstable or insufficient tear film, characteristic of dry eye syndrome, can temporarily create an irregular, uneven surface on the cornea. This dry surface scatters light and can lead to fluctuating blurriness or glare, especially noticeable when blinking. Another condition that increases night blurriness is corneal edema, or swelling of the cornea, which causes the light to bend irregularly upon entry.
When to Consult an Optometrist
If night blurriness is a long-standing, stable issue, it may simply require an updated prescription for corrective lenses to manage the refractive error and night myopia. However, certain symptoms should prompt an immediate and thorough eye examination. The most concerning signs are a sudden or rapid onset of vision changes, or night blurriness that significantly interferes with daily activities, particularly driving.
The appearance of fixed, colored, or rainbow-like halos around lights, especially if accompanied by severe eye pain or headache, can indicate a medical emergency such as acute angle-closure glaucoma. An eye doctor can determine if the underlying cause is manageable with simple solutions, like lubricating eye drops for dry eye, or if it requires more involved treatment such as cataract surgery. A comprehensive eye exam will ensure that any underlying conditions are identified and addressed promptly.