The phenomenon of rapid eye movement (REM) during sleep is one of the most distinctive aspects of the human sleep cycle. While the body rests, the eyes dart back and forth beneath the eyelids, a pattern that has fascinated scientists since its discovery. This unique activity is a direct result of intense neurological signaling deep within the brain. Understanding why our eyes move requires exploring the specific stage of sleep in which it occurs and the spontaneous electrical activity that drives the eye muscles.
The Architecture of Sleep
Human sleep follows a predictable, repeating pattern that cycles between two main states: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further divided into three stages (N1, N2, and N3), representing a progression from light dozing to the deepest, slow-wave sleep. During NREM stages, heart rate and breathing slow, body temperature drops, and brain waves become progressively slower.
The entire sleep cycle typically lasts between 90 and 110 minutes, with a person completing four to six cycles nightly. NREM sleep accounts for approximately 75 to 80 percent of this time.
REM sleep is the final stage in the cycle, and it is physiologically distinct from NREM stages, often referred to as “paradoxical sleep.” While the body is at its most relaxed, the brain becomes highly active, with brain waves resembling those of a waking state. The first REM period is usually short, lasting about 10 minutes, but these periods lengthen progressively as the night continues, dominating the final hours of sleep.
Neural Activity Driving Rapid Eye Movement
The physical movement of the eyes during REM sleep is a byproduct of specific, spontaneous bursts of electrical activity originating in the brainstem. This activity is known as Ponto-Geniculo-Occipital (PGO) waves, named for the three brain structures they sequentially activate: the pons, the lateral geniculate nucleus of the thalamus, and the occipital cortex. PGO waves are considered a neurological signature of REM sleep.
These waves arise from a cluster of neurons in the pons, a part of the brainstem. The neural signals travel to the oculomotor nuclei, which directly control the extrinsic eye muscles. The neurotransmitter acetylcholine is heavily involved in initiating this activity, contributing to the activated state of the brain during REM.
During REM sleep, muscle atonia causes a near-complete paralysis of the major skeletal muscles, which prevents us from acting out dreams. However, the muscles that control breathing and the muscles that move the eyes are uniquely exempt from this inhibition. This exemption allows the eye muscles to respond to the intense, random neural signals of the PGO waves, resulting in the rapid, darting movements that define the stage.
Eye Movement and Dreaming: Correlation vs. Causation
The rapid eye movements occur simultaneously with the period of sleep most strongly associated with vivid, narrative dreaming. This correlation led to the popular theory that the eyes are actively scanning or “watching” the events unfold in the dream world. However, scientific consensus views the movements as an effect rather than a functional tracking behavior.
The eye movements are best understood as the physical manifestation of spontaneous neural discharges from the brainstem. PGO waves stimulate the visual system and motor control centers, bombarding the cortex with internal signals that the brain attempts to interpret. This internal activation may contribute to the highly visual and hallucinatory nature of REM dreams.
Studies attempting to match the direction of an eye movement to a specific dream event have yielded inconsistent results, suggesting the movements are largely random bursts of energy. Furthermore, vivid dreaming can occur in NREM sleep, and some awakenings from REM sleep yield no dream report. This demonstrates that the two phenomena are correlated but not strictly causal. The rapid eye movements are more likely an efferent copy—a signal from the brain’s internal activity—rather than a purposeful sensory action.