The experience of seeing bright lights, colors, or patterns when your eyes are closed is a common visual phenomenon. These spontaneous visual perceptions, known as phosphenes, are generated entirely within the visual system without any external light source. While often harmless and temporary, phosphenes can occasionally signal a medical condition that warrants professional attention. Understanding the mechanisms behind these internal light shows helps distinguish between a benign occurrence and a symptom requiring immediate care.
Phosphenes Explained
Phosphenes are the perception of light in the absence of actual light, meaning the source of the sensation is an internal trigger, not external photons. The visual system—composed of the retina, optic nerve, and visual cortex—is designed to interpret any electrical or mechanical input as light. Photoreceptor cells in the retina, normally activated by light, can be stimulated by other forms of energy.
When these cells are disturbed, they generate the same electrical signal as if light were present. This signal travels along the optic nerve to the visual cortex, which translates the impulse into the perception of light. This mechanism explains why different stimuli, from a gentle rub to a physiological disturbance, result in the sensation of seeing flashes.
Common Causes of Phosphenes
The most frequent and non-concerning cause of phosphenes is mechanical pressure applied directly to the eyeball. Rubbing your eyes vigorously, or pressing on them gently, produces temporary bright, swirling light patterns, sometimes called “pressure phosphenes.” The force physically stimulates the light-sensitive rods and cones in the retina, causing them to fire an artificial signal to the brain.
Phosphenes can also be triggered by sudden changes in blood pressure or head position. Momentarily “seeing stars” after standing up too quickly, sneezing violently, or coughing hard is usually due to a transient drop in blood flow or increased pressure within the head. Sleeping with the face pressed firmly into a pillow can also generate phosphenes. These types of flashes are typically brief, disappearing within seconds, and are considered normal physiological responses.
When Flashes Signal a Medical Concern
Flashes of light that are persistent, recurrent, or sudden in onset are clinically referred to as photopsia, and they can indicate a problem within the posterior segment of the eye.
Posterior Vitreous Detachment (PVD)
One common cause is a Posterior Vitreous Detachment (PVD), an age-related process where the vitreous humor—the clear, gel-like substance filling the eyeball—begins to shrink and pull away from the retina. As the vitreous tugs on the retina, the mechanical stimulation creates flashes of light, often described as brief streaks or lightning in the peripheral vision. This process is common, occurring in a majority of people over the age of 60, but it requires an examination to ensure the retina is not damaged.
Retinal Tears and Detachment
A more serious concern is a retinal tear or a full Retinal Detachment, which may begin with symptoms similar to PVD. The repeated pulling of the vitreous can sometimes cause a break in the retinal tissue, which is a sight-threatening emergency. Flashes associated with this may be more frequent or intense, often accompanied by a sudden, significant increase in floaters, which appear as specks, strings, or cobwebs in the vision.
Migraine with Aura
Flashes may also be an early sign of a neurological event, such as a migraine with aura, which typically affects both eyes. These visual disturbances often manifest as a shimmering, expanding arc of jagged, zigzag lines, sometimes called a fortification spectrum, that grows and moves across the visual field. Migraine auras usually last between 10 and 30 minutes and may or may not be followed by a headache.
Any new onset of flashes, especially if they are persistent, intense, or associated with a dark shadow or “curtain” obscuring part of the vision, necessitates immediate attention from an eye specialist to rule out a retinal detachment.