Night myopia is a specific vision condition where distant objects appear blurry in low-light environments, such as at dusk or after nightfall. It means that while a person might have clear vision during the day, their eyesight for faraway objects deteriorates significantly in dim settings. This condition is distinct from general nearsightedness, as it is primarily triggered by changes in light conditions rather than a constant refractive error.
Why Vision Changes in the Dark
The human eye adapts to varying light levels, and in darkness, the pupil naturally dilates to allow more light to enter. This increased opening of the pupil, however, can introduce optical distortions known as spherical aberration. Spherical aberration causes light rays passing through the periphery of the dilated pupil to focus at a different point than those passing through the center, leading to an incorrect overall focus and perceived blurriness.
Furthermore, the eye’s sensitivity to different wavelengths of light shifts in low-light conditions, a phenomenon known as the Purkinje effect. During the day, cone cells are dominant and are most sensitive to yellow-green light. In dim light, rod cells become more active and are most sensitive to blue-green light. This shift in peak sensitivity, combined with the eye’s natural chromatic aberration—where different colors of light focus at slightly different points—can contribute to a myopic shift, making objects appear less clear. The eye’s accommodative system may also struggle to maintain a precise focus for distant objects in the absence of clear visual cues, leading to a tendency to over-accommodate, which further induces a myopic state.
Recognizing the Symptoms of Night Myopia
A common symptom is a noticeable blurriness of distant objects, such as road signs or street numbers, especially when driving after sunset. People might also find it challenging to discern details in dimly lit indoor environments, like theaters or restaurants, where their distance vision becomes less sharp than during daylight hours. Another frequent complaint involves visual phenomena around light sources. Many describe seeing halos, which are circles of light, or starbursts, fine lines radiating from bright lights like headlights or streetlights. These visual disturbances can make nighttime driving particularly challenging. Those who have a mild, uncorrected degree of nearsightedness during the day are often more prone to noticing these symptoms, as the effects of night myopia can exacerbate their existing refractive error.
Corrective Lenses and Driving Safety
Managing night myopia often involves specific corrective lenses. An optometrist can determine if a slightly different prescription, typically with additional minus power, would improve nighttime vision. This specialized correction helps to offset the myopic shift that occurs as the pupil dilates in dim light, allowing light to focus more accurately on the retina.
Lenses with an anti-reflective (AR) coating offer a significant advantage for those with night myopia. This coating minimizes glare and internal reflections that can be caused by headlights and streetlights, reducing the appearance of halos and starbursts. By allowing more light to pass through the lens to the eye, AR coatings enhance overall visual clarity and contrast during nighttime driving. Beyond corrective eyewear, practical safety measures for nighttime driving include regularly cleaning windshields and headlights to maximize light transmission. Reducing driving speed at night also provides more time to react to potential hazards.