Closing one eye while focusing on text is a common behavior indicating the visual system is under strain. This action is a coping mechanism the brain uses to overcome inefficiency in how the two eyes work together during near tasks. Reading monocularly, or with a single eye, signals an underlying binocular vision problem. This complex coordination allows both eyes to form a single, comfortable image. The body attempts to reduce strain and achieve a clearer, more stable view of the page.
Why Closing One Eye Provides Relief
The act of reading requires the brain to seamlessly merge two slightly different images into a single, three-dimensional perception known as binocular fusion. This fusion is necessary for depth perception and comfortable viewing. When the eyes struggle to align or focus, the brain receives conflicting signals, which can manifest as blurred vision, eye strain, or intermittent double vision (diplopia).
Closing one eye immediately eliminates the confusing input from the non-dominant or misaligned eye. Switching to monocular vision means the brain no longer has to reconcile two disparate images. This instantly reduces the processing load and the muscular effort required to maintain alignment, providing immediate relief. This allows the person to continue reading with less perceived effort, even though the underlying issue remains.
Ocular Muscle Imbalance and Coordination Problems
A frequent cause of closing one eye is a problem with eye teaming, specifically a binocular vision disorder like convergence insufficiency (CI). When reading, the eyes must turn inward, or converge, to maintain focus on the close-up text. CI is a condition where the eyes have difficulty maintaining this inward movement and tend to drift outward.
To prevent double vision, the brain triggers the eye muscles to exert extra effort to pull the eyes back into alignment. This strenuous effort leads to symptoms like eyestrain, headaches, and sleepiness during close work. Closing one eye immediately halts this struggle, bypassing the convergence demand entirely, and is a common behavioral symptom of CI. This issue is often missed in standard eye exams because visual acuity may be excellent, but the eyes lack the stamina and coordination for sustained near tasks.
Unmatched Focusing Power
Another reason for monocular reading relates to differences in image clarity between the two eyes, independent of muscle alignment. This issue often stems from mismatched refractive errors, such as anisometropia, where the two eyes require significantly different prescriptions. A difference of just one diopter or more between the eyes can cause visual discomfort.
If one eye has a much higher degree of refractive error than the other, the brain receives one clear image and one substantially blurred image. The brain finds it challenging to fuse images of such unequal clarity, leading to visual strain. To resolve this, the visual system may suppress the blurry image, or the individual will manually close the eye with the poorer focus to avoid visual confusion.
When to Seek Professional Evaluation
The act of closing one eye while reading signals that the visual system is under stress, not just a benign quirk. If this behavior is accompanied by persistent headaches, eye fatigue, or noticeable double vision, a comprehensive evaluation is warranted. A specialized eye exam is necessary to test for binocular vision disorders that are frequently overlooked in basic screenings.
The behavior itself is a symptom, and the underlying problem will continue to cause strain and may worsen over time without intervention. Diagnosing the specific cause, whether it is convergence insufficiency or a mismatched refractive error, allows for targeted treatment. Treatment pathways may involve specialized eye exercises (vision therapy), the use of prism lenses to assist with alignment, or an updated glasses prescription to manage the difference in focusing power.