Seeing a bright light, flash, or vivid geometric shape when falling asleep or waking up can be a startling experience. These momentary visual disturbances are generally a sign of the brain transitioning between different states of consciousness. While the experience can feel incredibly real, it is frequently a benign phenomenon resulting from normal, temporary changes in neurological activity.
Seeing Light During Sleep Transition
The most common explanation for seeing light flashes involves the brain’s temporary confusion during the process of falling asleep (hypnagogic state) or waking up (hypnopompic state). During these brief transitional periods, the brain can generate vivid sensory experiences known as hallucinations, which are not based on external reality.
These visual hallucinations are often simple, consisting primarily of bright colors, flashes of light, or geometric patterns. This occurs because the visual processing centers of the brain remain partially active while the rest of the brain begins to shut down for sleep. The brain misinterprets this residual neural activity as actual sensory input, leading to the perception of light or shapes.
Hypnagogic and hypnopompic hallucinations are considered normal phenomena and may affect many people at some point. They are not considered a disorder unless they become frequent or are accompanied by excessive daytime sleepiness. Factors such as chronic stress, fatigue, or a disrupted sleep schedule can increase the frequency of these transitional hallucinations.
Neurological Events During Sleep
Specific neurological events that occur during sleep can also cause the perception of bright light, often alongside other intense sensory input. One condition is Exploding Head Syndrome (EHS), a type of parasomnia occurring during sleep transition. EHS is primarily characterized by the perception of a sudden, extremely loud noise, such as a gunshot or a crashing sound, that only the person hears.
This intense auditory event is often accompanied by a concurrent visual disturbance, typically a flash or burst of bright light. EHS is thought to result from a delay in the deactivation of the brain’s reticular formation, which coordinates the transition to sleep. Instead of shutting down smoothly, sensory neurons appear to fire simultaneously, creating the sudden sensory input of both sound and light.
Another distinct neurological cause is a migraine aura, which can occur while sleeping or immediately upon waking. Auras are transient neurological symptoms often preceding or accompanying a migraine headache. The visual symptoms, referred to as scotomas, manifest as shimmering, zigzagging lines or bright, expanding spots of light that may move across the visual field. These phenomena are caused by a slow wave of electrical activity moving across the visual cortex of the brain.
Physical and Ocular Causes
Beyond the brain’s sleep-wake cycles, the physical structure of the eye itself can sometimes generate the perception of light. These flashes, known as phosphenes, are created when the retina’s light-sensitive cells are stimulated by mechanical pressure rather than actual light. If a person sleeps face down or presses their hand against a closed eye, the pressure can directly stimulate the retina and optic nerve.
This mechanical stimulation causes the retinal cells to fire electrical signals, which the brain interprets as flashes, streaks, or diffuse colored patches of light. Phosphenes are typically brief and disappear quickly once the pressure is relieved.
More serious ocular issues can also cause flashes of light, a symptom known as photopsia, which may be noticed upon waking. As the eye ages, the vitreous gel filling the eyeball naturally shrinks and pulls away from the retina, a process called posterior vitreous detachment. If the gel remains attached in certain spots, it can tug on the retina, causing the light-sensitive tissue to fire signals. These flashes are often described as lightning streaks or shooting stars, and they are typically more frequent and persistent than the brief flashes of a sleep hallucination.
When to Seek Medical Guidance
While most sleep-related light flashes are benign, certain associated symptoms require medical evaluation. The sudden appearance of new, persistent flashes, especially those not tied only to falling asleep or waking up, should prompt a consultation with an eye care professional. This is important if the flashes are accompanied by a sudden increase in floaters, which are dark spots or threads drifting across the vision.
A medical assessment is also advisable if the visual disturbances involve a change in the field of vision. Examples include a shadow or a curtain-like effect moving across the eye, which may signal a retinal tear or detachment. Additionally, if the light flashes are associated with severe pain, confusion, or new neurological symptoms like numbness or weakness, a doctor should be consulted.