Falling asleep triggers physiological shifts in the body and brain, and the eyes are no exception to this transformation. While the eyelids typically close to shield the eye from the environment, the eyeball itself is not simply static or resting during the hours of unconsciousness. Instead, the eyes undergo specific, patterned movements and positional changes that correspond directly to the different phases of the sleep cycle. Understanding these dynamics provides insight into the protective reflexes and active brain states that occur beneath the closed lids throughout the night.
Eye Position During Non-REM Sleep
During the non-rapid eye movement (NREM) stages, which constitute the majority of sleep time, the eyes tend to assume a relaxed and distinct orientation. As the body settles into the deeper stages of NREM sleep, the muscles controlling eye movement generally relax, allowing the eyeballs to drift. This relaxed state often results in a slow, wandering movement or a static position that differs from the active motion seen later in the cycle.
A highly specific phenomenon called Bell’s phenomenon often dictates the eye’s position during this time. This is a natural, protective reflex that causes the eyeball to rotate upward and slightly outward when the eyelids close. The purpose of this movement is to tuck the vulnerable, transparent front surface of the eye, the cornea, safely under the upper eyelid. This positioning protects the delicate surface from minor trauma and shields it from drying out. This upward and outward roll is present in approximately 75% to 80% of the population.
The Characteristics of Rapid Eye Movement
The phase known as rapid eye movement (REM) sleep is defined by a dramatic shift in both brain activity and the corresponding movement of the eyes. During REM, the brain becomes highly active, resembling an awake state, which is why this period is often referred to as paradoxical sleep. This brain activity is associated with the experience of vivid dreaming, and the eyes begin to move rapidly beneath the closed lids.
The movements are quick, jerky, and darting, lasting for short bursts as the eyes appear to scan an internal scene. These movements are not random muscle twitches but are thought to be tied to the visual and spatial content of the dream itself. Research suggests the direction and amplitude of the rapid eye movements may reflect perceived gaze shifts within the sleeper’s virtual dream environment.
This active state is a clear contrast to the slow, rolling drift or static position of the NREM phases that precede it. The presence of these distinctive movements is so consistent that it gives the entire stage its name. The brainstem region known as the paramedian pontine reticular formation is responsible for generating these bursts of rapid eye motion, even though the rest of the body’s major skeletal muscles are temporarily paralyzed.
When Eyes Remain Partially Open
In some cases, the eyes do not fully close during sleep, a condition specifically termed nocturnal lagophthalmos. This phenomenon results in the eyelids failing to completely cover the eyeball, leaving a small gap or slit. The primary causes of this partial opening are often related to issues with the seventh cranial nerve, which controls the muscle responsible for closing the eyelids, or structural factors, such as the shape of the eyelids or the protrusion of the eyeball.
When the eye remains exposed, even slightly, it bypasses the protective function of the closed lid. This exposure allows the tear film, which normally lubricates and protects the eye, to evaporate at an increased rate. Consequently, individuals with this condition frequently wake up with symptoms such as dry, gritty, or irritated eyes. The exposed cornea is more vulnerable to drying out overnight, which can lead to redness and discomfort upon waking. The inability of the eyelids to fully meet is a distinct physiological variation from the typical appearance of fully closed eyes during sleep.