A person’s eyes may move beneath their closed eyelids during sleep. These movements are a normal part of the sleep cycle, occurring during specific phases of rest. They indicate an active period within the sleeping brain. Understanding these eye movements helps demystify this aspect of our nightly unconscious state.
The Stages of Sleep
Sleep cycles through distinct phases: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. A typical sleep cycle lasts about 90 to 120 minutes, with individuals experiencing four to five cycles each night. The initial stages, NREM, are characterized by a gradual slowing of bodily functions.
NREM sleep progresses through three stages, from light to deep sleep. During NREM stages, eye movements are minimal or absent; the eyes remain still. This period is when the body undertakes restorative processes, such as repairing tissues, consolidating memories, and strengthening the immune system. Following NREM stages, the brain transitions into REM sleep.
Why Eyes Move During REM Sleep
Rapid eye movement (REM) sleep is named for the characteristic darting eye movements. This stage is often called “paradoxical sleep” because, despite the body’s temporary muscle paralysis, brain activity resembles wakefulness. Most vivid dreams occur during REM sleep, and eye movements connect to these visual experiences.
The “scanning hypothesis” suggests that these eye movements are not random but reflect a person’s gaze shifting within their dream environment, similar to how eyes move when scanning a scene while awake. Research supports this idea, showing eye movement direction can align with perceived changes in a dream’s virtual world. These movements are involuntary, arising from specific brain activity, particularly electrical bursts called ponto-geniculo-occipital (PGO) waves that originate in the brainstem.
This intense brain activity during REM sleep, including eye movements, is part of the brain’s process of organizing and consolidating memories and emotional regulation. The brain generates internal visual information, and the eyes may “track” these internal images as part of this neurological process. Eye movements serve as an external sign of the internal processing occurring during this sleep stage.
When Eye Movements Might Indicate a Problem
While eye movements during REM sleep are a normal physiological process, certain patterns or associated behaviors might signal an underlying concern. Normal REM eye movements differ from other involuntary eye movements, like nystagmus, which occur during wakefulness and can indicate neurological issues. The primary sleep-related condition where eye movements, combined with other physical actions, become problematic is REM sleep behavior disorder (RBD).
In normal REM sleep, the body experiences temporary paralysis, preventing individuals from acting out dreams. With RBD, this muscle paralysis is absent or incomplete, allowing a person to physically act out their vivid dream content. These actions can range from simple twitches to more complex behaviors like shouting, punching, kicking, or even jumping out of bed, sometimes resulting in injury.
RBD can occur suddenly and may worsen over time, often appearing after age 50. It can be an early indicator of certain neurodegenerative conditions, such as Parkinson’s disease or Lewy body dementia, sometimes appearing years or decades before other symptoms. If someone experiences dream-enacting behaviors, especially if violent or disruptive, a medical evaluation by a healthcare professional or sleep specialist is advisable. A sleep study, polysomnography, can help diagnose RBD by monitoring brain activity, eye movements, and muscle activity during sleep to determine if the normal paralysis is absent.