Seeing flashes, spots, or shimmering patterns when your eyes are closed in a dark room is a common and often unsettling phenomenon. This perception of light without any external light source is scientifically termed a phosphene. Understanding the mechanisms behind these internal light shows can demystify the experience, which is usually a harmless function of the visual system. Phosphenes occur because the retina and the brain’s visual cortex can be stimulated by factors other than light, causing the visual system to generate its own signals.
What Exactly Are Phosphenes
Phosphenes are visual sensations that originate within the eye or brain, bypassing the need for light to enter the pupil. These internal lights are a form of photopsia, which is a blanket term for the perception of flashes.
The appearance of phosphenes is typically simple, often described as glittery sparkles, random dots, squiggles, or basic geometric shapes. They may be white, colored, or patterned, and they often move across the visual field. This quality differentiates them from floaters, which are physical debris within the vitreous humor that cast shadows on the retina, appearing as shadowy specks that drift with eye movement. Complex, detailed images are considered visual hallucinations, which are distinct from the simple light patterns of phosphenes.
Benign Causes of Light in the Dark
The most frequent reasons for seeing phosphenes while settling down to sleep are entirely harmless and relate to the physical and electrical activity of the eye itself.
Mechanical Stimulation
One common cause is mechanical stimulation, often called a pressure phosphene. If you rub your eyes or press on your eyelids, the physical force activates the photoreceptor cells in the retina. This mechanical action mimics the energy of actual light, causing the retina to send an electrical signal to the brain, which is then interpreted as a flash of light or color.
Intrinsic Electrical Noise
Another cause is the intrinsic electrical noise of the visual system, sometimes referred to as “dark noise” or Eigengrau. Photoreceptors and associated neurons spontaneously fire electrical impulses at a low, baseline rate, even in total darkness. The brain interprets this random neural activity as a faint, grainy, or shimmering field of light. This spontaneous firing becomes more noticeable when the eyes are closed for an extended period.
Sleep Transition and Fatigue
Phosphenes can also be linked to the transition phase between wakefulness and sleep, known as hypnagogia. As the brain begins to shut down external processing, the visual cortex may generate simple, abstract patterns. Rapid eye movements beneath the eyelids, which occur during early sleep stages, can also physically stimulate the retina. General fatigue and eye strain can increase the sensitivity of the visual system to these internal stimuli, making phosphenes more pronounced as you try to rest.
When Phosphenes Indicate a Serious Problem
While most phosphenes are benign, a sudden onset of flashes or persistent light perceptions can indicate a more serious underlying condition.
Vitreous Traction and Detachment
Phosphenes that appear as bright, sudden flashes or streaks of light, especially in the side vision, may signal vitreous traction. The vitreous humor, the gel that fills the eyeball, naturally shrinks and liquefies with age. If it pulls on the light-sensitive retina, it causes the perception of a flash. This is known as a posterior vitreous detachment. While typically harmless, strong pulling can cause a retinal tear or detachment.
A retinal tear or detachment is a medical emergency where the retina pulls away from its underlying supportive tissue. In this situation, the flashes are usually brief, occurring repeatedly, and may be accompanied by a sudden increase in floaters, described as a shower of dots. The most concerning symptom is the appearance of a dark shadow or a curtain-like obstruction moving across the field of vision, which indicates a loss of sight due to the detachment.
Ocular Migraines
Phosphenes can also be a component of an ocular migraine, which involves visual disturbances that are neurological in origin. Migraine-related phosphenes often manifest as a visual aura, typically affecting both eyes. These auras are characterized by complex, shimmering, zigzag lines or arcs of light that gradually expand across the visual field over a period of 5 to 60 minutes. Unlike flashes from retinal issues, migraine auras are generated in the brain’s visual cortex and tend to be more structured and temporary. Any sudden or dramatic change in the frequency, brightness, or type of phosphene, particularly if accompanied by vision loss or a substantial increase in floaters, warrants an immediate consultation with an eye care professional.