Seeing bright lights at night often leads to questions about why they appear surrounded by rings, streaks, or a fuzzy glow. Understanding how the human eye functions in low-light conditions, known as scotopic vision, reveals that this process is fundamentally different from how we see during the day. This difference in visual mechanics is the primary reason why light sources can seem distorted after the sun goes down. The severity of halos and glare can vary widely from person to person.
The Mechanics of Seeing in Low Light
The ability to see in darkness relies heavily on specialized light-sensitive cells in the retina called rods. Unlike cones, which handle color and fine detail in bright light, rods are far more numerous and sensitive to low levels of illumination. Rods use the photopigment rhodopsin, which allows the eye to adapt to the dark, a process that can take up to 30 minutes to achieve maximum sensitivity. In low light, the pupil dilates significantly to maximize the amount of light entering the eye. This dilated pupil allows light to pass through the entire lens and cornea, including the peripheral edges.
Understanding Normal Glare and Halos
The appearance of halos, glare, and subtle starbursts around light sources at night is, to a degree, a normal consequence of the eye’s physical structure and its adaptation to darkness. When the pupil expands, light travels through the outer zones of the lens and cornea. These peripheral areas often possess slight imperfections and curves that are not used in daytime vision when the pupil is small. As light passes through these peripheral areas, it is scattered and diffracted, rather than being focused precisely on the retina. This scattering creates the perception of a halo or an extended glow around the light source itself.
This normal optical scattering is why people with larger natural pupils tend to notice these effects more intensely than those with smaller pupils. The phenomenon is further intensified by the fact that rods, which are primarily active at night, do not provide the sharp visual acuity that cones offer during the day. The resulting image is inherently fuzzier and less defined, making any light scattering more noticeable.
Medical Conditions That Distort Night Sight
A significant or worsening increase in night vision distortions often points to an underlying ocular condition. One of the most common causes is astigmatism, a refractive error where the cornea or lens is shaped more like a football than a basketball. This irregularity prevents light from focusing evenly on the retina, causing light sources to appear streaky or hazy, particularly when the pupil is dilated at night.
Cataracts, which involve the clouding of the eye’s natural lens, are another primary contributor to excessive glare and halos. The cloudy lens scatters incoming light dramatically, significantly reducing contrast and making bright lights appear much more dazzling and disruptive. Dry eye syndrome can also worsen night vision disturbances, as an inadequate tear film creates an irregular surface on the cornea. This uneven surface scatters light before it even enters the eye, leading to increased glare and reduced clarity. Patients who have undergone refractive surgeries, such as LASIK, may also experience temporary or sometimes persistent night vision disturbances. The reshaping of the cornea can induce higher-order aberrations, which are complex vision errors that scatter light and cause halos or starbursts.
Strategies for Improving Nighttime Vision
Addressing significant night vision problems begins with a comprehensive eye examination to diagnose any underlying medical conditions. If the issue stems from a refractive error like astigmatism, updating the prescription in glasses or contact lenses can often provide substantial relief. For those who wear glasses, anti-reflective (AR) coatings are a simple yet effective solution. The coating minimizes reflections and glare from both the front and back surfaces of the lenses, allowing up to 99.5% of available light to pass through to the eye. Maintaining clean windshields and keeping eyeglasses smudge-free also reduces extraneous light scattering that exacerbates night distortions. Persistent or acutely worsening night vision symptoms, especially if accompanied by pain or sudden visual changes, should prompt an immediate consultation with an eye care specialist.