Those flashes, sparkles, and swirling lights you see behind closed eyelids are usually phosphenes, visual sensations produced not by light entering the eye but by spontaneous or stimulated activity in cells along your visual pathway. Phosphenes can be triggered by mechanical pressure, electrical signals, and even random neural firing in the retina and brain. Most of the time they are completely harmless, but in some cases they signal something that needs medical attention.
What a Phosphene Actually Is
Your visual system is always active, even in total darkness. Retinal cells and neurons farther along the visual pathway fire at low levels whether or not light is hitting them. When that baseline firing happens to synchronize or spike, your brain interprets the signal the same way it would interpret real light, and you perceive a flash, dot, or swirl of color. These perceptions are called phosphenes, and they can be caused by mechanical pressure, electrical stimulation, magnetic fields, or simply the random firing of cells in the visual system.1ScienceDirect. Phosphene phenomenon: A new concept
The retina’s ganglion cells, the neurons that send visual signals to the brain, can fire in synchronized bursts even when no light stimulus is present. Research on primate retinas has shown that neighboring ganglion cells share strong synaptic input that can drive rapid synchronized firing in the absence of any visual signal.2SpringerLink. Modeling the impact of common noise inputs on the network activity of retinal ganglion cells In practical terms, that means your retina is not simply “off” when your eyes are closed. It is still generating signals, and some of those signals reach your visual cortex as brief flickers of light.
The Most Common Harmless Causes
If you press your palms against your closed eyes or rub them, you will almost certainly see patterns of light. This is a mechanical phosphene. The physical pressure deforms retinal cells enough to trigger the same electrical signal that light would. It is the most universal form of phosphene and essentially harmless, though rubbing your eyes aggressively and regularly is not great for the cornea.
Eye movements themselves can also generate faint flashes. When you move your eyes quickly, some mechanical tug is transmitted through the vitreous gel to the retina. In most people this is too subtle to notice, but in certain conditions the effect becomes more pronounced. Research on electrically induced phosphenes has confirmed that the perceived position of a flash can shift when subjects move their eyes to the periphery, though within normal, everyday eye movements the shift is small enough to go unnoticed.3PubMed Central. Evaluation of Phosphene Shifts During Eye Movements to Enhance Safe Visual Assistance for Visually Impaired Individuals
Another extremely common scenario is seeing flashes or geometric patterns as you fall asleep. These hypnagogic experiences, visual perceptions that occur during the transition between wakefulness and sleep, have been documented since antiquity. They can range from formless blobs of color to vivid, almost dreamlike images. The reverse can happen on waking, too. These are considered normal features of sleep-wake transitions, not hallucinations in the clinical sense, because they occur during a state that is neither fully awake nor fully asleep.4Schizophrenia Bulletin. What Is the Link Between Hallucinations, Dreams, and Hypnagogic–Hypnopompic Experiences?
When the Vitreous Gel Is the Culprit
The interior of your eye is filled with a clear, jelly-like substance called the vitreous. As you age, this gel gradually liquefies and can start to pull away from the retina, a process called posterior vitreous detachment, or PVD. That tugging mechanically stimulates retinal cells and produces flashes of light. A large review of patients reporting flashing lights found PVD was the single most common cause, responsible for about 40% of cases.5ScienceDirect. Photopsias: A Key to Diagnosis
PVD flashes have a distinctive profile. They tend to be quick, lightning-bolt-shaped, white, and located in the outer (temporal) part of your visual field. Most people notice them more in the dark than in lit environments, and head or eye movements often trigger them. New-onset floaters, those drifting spots and strands, frequently accompany the flashes.5ScienceDirect. Photopsias: A Key to Diagnosis PVD itself is usually benign and happens to most people eventually, often by their sixties or seventies. Nearsighted people tend to experience it earlier.
The concern with PVD is that the pulling vitreous can sometimes tear the retina. In the same review, retinal tears accounted for about 9% of photopsia cases, and full retinal detachments accounted for roughly 8%.5ScienceDirect. Photopsias: A Key to Diagnosis Flashes and floaters are considered the hallmark warning symptoms for these conditions, and their sudden onset warrants prompt evaluation because a retinal tear caught early can often be sealed with a laser before it progresses to a detachment.6PubMed. Symptoms related to posterior vitreous detachment and the risk of developing retinal tears: a systematic review
Migraine Aura and Cortical Spreading Depression
Flashing lights that look like shimmering zigzag lines, expanding arcs, or a flickering “fortification spectrum” are classic migraine aura. Unlike the brief lightning-bolt flashes of PVD, migraine visual disturbances usually develop over five to thirty minutes, spread across one half of the visual field, and then fade. They are generated not in the eye but in the visual cortex at the back of the brain, driven by a wave of abnormal electrical activity called cortical spreading depression.7PubMed Central. Persistent and Repetitive Visual Disturbances in Migraine: A Review
Some people experience migraine aura without any headache at all, a phenomenon sometimes called “silent migraine” or “acephalgic migraine.” This can be confusing because the visual symptoms seem to come out of nowhere. In the photopsia review mentioned earlier, classic and ophthalmic migraine together accounted for about 7% of all photopsia presentations.5ScienceDirect. Photopsias: A Key to Diagnosis The key distinguishing features are the gradual buildup, the geometric quality of the visuals, and the fact that they appear in both eyes simultaneously because the source is cortical rather than retinal.
Drug-Induced Phosphenes
Some medications can directly trigger flashes of light by interfering with the electrical activity of retinal cells. The best-studied example is ivabradine, a heart-rate-lowering drug prescribed for angina and heart failure. Ivabradine works by blocking a specific type of ion channel in heart cells, but a similar channel exists in retinal neurons. When the drug blocks those retinal channels, it can produce phosphenes in some patients.8PubMed Central. Cellular mechanisms underlying the pharmacological induction of phosphenes The flashes tend to be brief, occur in low-light conditions, and are considered a recognized side effect rather than a danger sign.
Research into the mechanism has shown that blocking these channels in the retina disrupts the balance between different visual processing pathways, which is thought to create abnormal signals the brain reads as light.9PubMed. Differential Effects of HCN Channel Block on On and Off Pathways in the Retina as a Potential Cause for Medication-Induced Phosphene Perception Digitalis, a much older heart medication, was also identified among photopsia causes in the clinical review, though it accounted for less than 1% of cases.5ScienceDirect. Photopsias: A Key to Diagnosis If you have recently started a new medication and notice flashing lights, it is worth mentioning to your prescriber rather than assuming the worst.
Visual Snow Syndrome
Some people see flickering dots across their entire visual field at all times, a bit like the static on an old television set. This is visual snow syndrome, and it involves a constant, involuntary visual disturbance rather than the brief flashes most people associate with phosphenes. The condition was barely recognized in the medical literature until recently, with fewer than ten published cases before 2014 and roughly 200 cases reported in the few years that followed.10PubMed Central. Visual Snow Syndrome: Proposed Criteria, Clinical Implications, and Pathophysiology
Visual snow is thought to arise from cortical hyperexcitability, meaning the brain’s visual processing areas are firing more actively than they should. Many patients also experience other visual phenomena like afterimages that linger too long, trails behind moving objects, and photophobia. The condition is often confused with migraine aura, but while migraineurs have episodes, visual snow sufferers report their symptoms around the clock. For anyone who has always seen a fine layer of “static” with eyes open or closed, visual snow is the most likely explanation, and it has nothing to do with the retina.
Optic Neuritis and Nerve Inflammation
People with optic neuritis, inflammation of the optic nerve often associated with multiple sclerosis, can experience a distinctive type of phosphene triggered by eye movement. These flashes differ from the normal brief sparkles healthy people sometimes notice. They share characteristics with the Lhermitte sign, a phenomenon in MS where bending the neck sends an electric-shock sensation down the spine, and are believed to result from a similar mechanism of demyelinated nerve fibers firing abnormally when mechanically disturbed.11PubMed. Movement phosphenes in optic neuritis: a new clinical sign
These phosphenes are not caused by vitreous or retinal defects, which makes them clinically interesting. If you notice that flashes reliably appear every time you move your eyes in a certain direction and you also have symptoms like blurry vision, pain with eye movement, or color desaturation in one eye, optic neuritis is a possibility worth investigating.
Occipital Epilepsy
Seizures originating in the occipital lobe, the brain’s visual processing center, can produce flashing lights as their primary symptom. In a systematic study of 18 patients with symptomatic occipital epilepsy, elementary visual hallucinations mainly consisted of small, colored, circular patterns that flashed or multiplied in one half of the visual field.12PubMed. Visual phenomena and headache in occipital epilepsy: a review, a systematic study and differentiation from migraine More than half of the patients in that study had normal mental and neurological exams despite having visible brain lesions on imaging, which shows that occipital seizures can look deceptively mild from the outside.
Distinguishing occipital epilepsy from migraine aura can be tricky because both originate in the visual cortex. Epileptic visual hallucinations tend to be shorter, more stereotyped from episode to episode, and more likely to involve colored rather than achromatic patterns. Migraine aura, by contrast, typically evolves over minutes and has a more gradual buildup. When someone has recurrent, brief episodes of patterned flashing light, especially in a consistent part of the visual field, epilepsy should be on the list of things to consider.
Charles Bonnet Syndrome and Vision Loss
People who have lost substantial vision, from macular degeneration, glaucoma, or other causes, sometimes begin seeing vivid visual hallucinations that range from simple flashes and geometric patterns to complex images of people or landscapes. This is Charles Bonnet syndrome, a condition where visual hallucinations occur as a result of damage along the visual pathway.13PubMed Central. Hallucinations Experienced by Visually Impaired: Charles Bonnet Syndrome The currently accepted explanation is that vision loss leads to a form of sensory deprivation in the visual cortex, causing disinhibition and spontaneous firing in visual regions that no longer receive normal input.14Arquivos de Neuro-Psiquiatria. Charles Bonnet syndrome: characteristics of its visual hallucinations and differential diagnosis
Charles Bonnet syndrome is underreported because patients worry that describing hallucinations will lead to a psychiatric diagnosis. The hallucinations are understood to be a neurological phenomenon, not a sign of mental illness. They are the brain’s visual cortex essentially “filling in the blanks” when it stops receiving real signals. If you or an older relative with declining vision starts seeing things that are not there but can recognize them as unreal, this is a well-documented and benign condition worth discussing with an eye doctor rather than hiding out of embarrassment.
When to Seek Urgent Care
Most phosphenes are harmless, but a few patterns should prompt a same-day or next-day visit to an eye doctor:
- Sudden onset: A new burst of flashes, especially in one eye, with a shower of new floaters can indicate a retinal tear or detachment. The classic warning sign is something described as a curtain or shadow moving across part of your vision.
- Persistence: Flashes that continue for hours or days rather than seconds, or recur repeatedly over a short period, are more concerning than a single brief flash at bedtime.
- Association with vision loss: Any flash accompanied by a noticeable loss of peripheral or central vision is a potential emergency.
- Headache and neurological symptoms: Flashes accompanied by severe headache, confusion, weakness, or speech difficulty could point to a neurological event rather than an eye problem.
By contrast, the occasional faint sparkle or swirl you notice as you close your eyes in a dark room, or the brief shimmer you see when you stand up quickly, is overwhelmingly likely to be normal retinal activity, a mild pressure-related phosphene, or a brief dip in blood flow. The transient, bilateral, and non-progressive nature of these everyday flashes is what separates them from the warning signs above.
Cosmic Rays and Astronaut Light Flashes
One of the more striking demonstrations that phosphenes are not about light entering the eye comes from space. Astronauts have reported seeing flashes of light with their eyes closed since the Apollo missions. On the International Space Station, about 20 high-energy ions per minute pass through the crew’s eyes (excluding the lightest particles), and astronauts perceive some of these as brief streaks or dots of light.15New Journal of Physics. Heavy ions light flashes and brain functions: recent observations at accelerators and in spaceflight Laboratory work has confirmed that the radiation can activate rhodopsin, the light-sensitive protein in retinal cells, at the very start of the visual signaling cascade. So in this case, cosmic rays are literally stimulating the same molecular switch that normal photons flip.
Researchers have replicated the effect on the ground using heavy-ion particle accelerators, where subjects exposed to similar radiation reported comparable flashes. The phenomenon is relevant beyond space travel: patients undergoing heavy-ion therapy for cancer have also reported light flashes during treatment. It is a vivid illustration of how the visual system is not just a passive camera waiting for light but a neural circuit that can be activated by any sufficiently strong physical stimulus, whether that is pressure from your fingertip, an electrical current, or a charged particle from a distant star.
Transcranial Magnetic Stimulation and Artificial Phosphenes
Phosphenes can also be generated artificially by applying magnetic pulses to the back of the head, directly stimulating the visual cortex through the skull. In transcranial magnetic stimulation (TMS) studies, magnetic pulses applied over the occipital cortex reliably produce phosphenes and can even suppress the visibility of real visual stimuli when timed precisely. Parietal stimulation, interestingly, also produces phosphenes but does not consistently suppress real visual processing the way occipital stimulation does.16PubMed Central. Phosphene-guided transcranial magnetic stimulation of occipital but not parietal cortex suppresses stimulus visibility
This line of research matters far beyond the laboratory. Teams developing visual prosthetics for blind individuals are working on devices that use arrays of electrodes to create patterns of phosphenes, essentially spelling out the visual world in controlled flashes. The challenge is that phosphene positions shift when the eyes move, making it harder to create a stable “image.” Current work is focused on mapping how much the perceived flash location drifts with eye movement and figuring out compensation strategies that could make phosphene-based vision aids practical for everyday tasks like walking.