Sleep is divided into two main phases: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. The eyes do not simply stop moving when a person falls asleep; instead, their activity changes dramatically across these phases. Eye movement shifts from slow, coordinated drifts to bursts of rapid, involuntary motion, reflecting the changing state of the brain. Understanding eye behavior requires examining both the quiet, deep stages and the highly active, dreaming state.
Eye Position During Non-REM Sleep
Non-Rapid Eye Movement (NREM) sleep makes up roughly 75% of a night’s rest. During this phase, the eyes exhibit a distinct pattern of slow movement that eventually ceases. The initial stage, N1, is the brief transition from wakefulness to sleep, where the eyes typically roll slowly and uncoordinatedly beneath the closed eyelids.
As sleep deepens into the N2 stage, which is the largest portion of total sleep time, eye movement diminishes significantly, and the eyes settle into a quiet, fixed position. The deepest stage, N3, is often called slow-wave sleep due to the prominent slow delta waves dominating brain activity.
In the N3 stage, the eyes stop moving altogether, becoming still and resting in a position that is often slightly upward-tilted. This stillness reflects the overall reduced physiological activity of the body, including slower heart rate and breathing, allowing for physical restoration and tissue repair.
The Rapid Eye Movements of REM Sleep
Rapid Eye Movement (REM) sleep is defined by the sudden, darting motion of the eyes behind closed eyelids. These movements are fast, sporadic, and involuntary, often appearing as if the eyes are scanning a visual field.
REM sleep is often referred to as paradoxical sleep because the brain becomes highly active, resembling an awake state, while the body’s major muscles are temporarily paralyzed. This stage is strongly associated with vivid dreaming, and awakening a person during this phase often results in a detailed dream report. The eyes dart in multiple directions—side-to-side and up and down—in tandem with intense brain activity.
Why Our Eyes Move While We Dream
The physical eye movements observed during REM sleep are driven by spontaneous bursts of electrical energy originating in the brainstem, specifically in the pons. These bursts are known as Ponto-Geniculo-Occipital (PGO) waves.
PGO waves sequentially activate the pons, the lateral geniculate nucleus of the thalamus, and the occipital cortex. They are considered a neurological signature of REM sleep, directly stimulating the oculomotor nuclei that control the eye muscles.
This intense neural signaling activates the visual processing centers of the brain, even without external input, which is thought to be the source of dream imagery. The eyes move because the brainstem’s output bypasses the paralysis mechanism that inhibits the rest of the body’s musculature.
Theories on Eye Movement Function
The “Scanning Hypothesis” proposes the eyes are actively looking at the visual landscape of the dream world, with movements corresponding to shifts in attention within the dream narrative. Recent research supports this idea, showing that in mice, the direction of eye movements aligns with the activity of brain cells signaling the animal’s perceived “heading direction” in its virtual dream environment.
The “Maintenance Theory” views the movements as a side effect of intense brainstem activity. This theory posits that the rapid movements may not be functional for the dream itself, but could serve a homeostatic or developmental purpose. These purposes include exercising the oculomotor system or helping to maintain neural connections in the visual system. The high proportion of REM sleep in infants supports the idea that this phase is important for neural maturation and plasticity.