While humans typically close both eyes during sleep, the concept of sleeping with one eye open is not merely a figure of speech. This intriguing ability is a real biological phenomenon observed in many species across the animal kingdom, though it manifests differently in humans.
Unihemispheric Sleep in Animals
Many animals exhibit unihemispheric slow-wave sleep (USWS), a unique pattern where one half of the brain rests while the other remains active. During USWS, the eye connected to the sleeping hemisphere closes, while the eye linked to the awake hemisphere stays open and vigilant. Electroencephalography (EEG) studies show distinct slow-wave activity in one brain half and wakefulness patterns in the other.
This adaptation is present in various marine mammals, including dolphins, whales, and seals, as well as numerous bird species. Bottlenose dolphins utilize USWS, allowing one half of their brain to rest while the other remains alert. Northern fur seals display USWS when in water, contrasting with their bihemispheric sleep on land. Birds like frigatebirds engage in USWS during long flights.
Purpose of One-Eyed Sleep
Sleeping with one eye open provides significant survival advantages for animals. This partial alertness allows them to maintain vigilance for predators. It also enables them to continue physiological functions, such as swimming or flying, without fully losing consciousness.
For aquatic mammals like dolphins, USWS is important because breathing is a voluntary action, requiring them to surface regularly even while resting. In birds, this adaptation allows continued visual navigation and aerodynamic control during migratory flights, preventing collisions and maintaining flock cohesion. This allows animals to rest in challenging or high-risk environments.
Human Eye Closure During Sleep
Unlike animals that exhibit unihemispheric sleep, humans do not naturally possess this ability. During typical human sleep, both eyes close completely as the facial muscles relax. This full closure is important for lubricating the eye surface and blocking out light, which regulates the body’s internal clock.
Some individuals experience nocturnal lagophthalmos, a condition involving incomplete eye closure during sleep. This is a medical condition, not a natural adaptation. The eyelids may close partially, but do not seal completely, leaving a small opening. This condition affects an estimated 5% of the population.
When Eyes Don’t Fully Close
When a human’s eyes do not fully close during sleep, it has various causes. These include issues with facial nerves, such as those affected by Bell’s palsy, a stroke, or tumors, which weaken the muscles controlling eyelid movement. Eyelid abnormalities, like weak muscles, scarring from injuries or surgeries, or anatomical differences, also prevent complete closure. Conditions that cause the eyes to bulge forward, such as Graves’ disease, also contribute.
Incomplete eye closure can lead to symptoms including dry eyes, irritation, redness, or a sensation of having a foreign object in the eye. Without adequate lubrication, the eyes are more susceptible to infections, scratches on the cornea, or ulcers. If left unaddressed, these issues can lead to vision problems. Individuals experiencing these symptoms should consult a healthcare professional.