Sleep often brings to mind images of closed eyes and stillness. However, many animal species exhibit the remarkable ability to rest while keeping their eyes open. This counterintuitive form of slumber challenges the conventional understanding of sleep, revealing diverse adaptations that balance the need for rest with survival demands.
Animals Known to Sleep with Eyes Open
Many aquatic mammals, such as dolphins, whales, and seals, sleep with their eyes open. Dolphins and beluga whales can peacefully float or slowly swim with one eye open, maintaining awareness of their surroundings. This unique resting state helps them surface regularly for air. Seals, particularly northern fur seals, exhibit this behavior when in water, though they can also sleep with both eyes closed on land.
Bird species also frequently display open-eyed slumber, often adjusting their eye closure based on perceived threat levels. Ducks, for example, can sleep with one eye open, especially when on the perimeter of a flock, allowing them to monitor for danger. This vigilance is important for birds during long migratory flights, where short bursts of open-eyed sleep can occur.
Reptiles and fish commonly sleep with their eyes open due to anatomical differences. Most fish lack eyelids, so their eyes cannot physically close. Similarly, many reptiles, including snakes and some lizards like the bearded dragon, do not possess movable eyelids. Instead, their eyes are covered by a transparent scale called a “spectacle” or “brille,” which protects the eye.
Crocodilians, while capable of closing their eyes, often sleep with them partially open. Some amphibians, like bullfrogs, also lack eyelids and thus sleep with their eyes open.
The Science Behind Open-Eyed Sleep
The primary mechanism enabling open-eyed sleep in many animals is unihemispheric slow-wave sleep (USWS). This specialized form of rest allows one half of the brain to enter a deep sleep state characterized by slow brain waves, while the other hemisphere remains active and alert. The eye corresponding to the sleeping hemisphere is typically closed, while the eye connected to the awake hemisphere remains open. This ensures brain hemispheres alternate rest periods, receiving adequate sleep over time.
USWS is a type of non-rapid eye movement (NREM) sleep. In animals capable of USWS, NREM sleep occurs in only one hemisphere at a time. The brain can effectively reduce sensory processing in the resting hemisphere while maintaining vigilance in the active one.
For animals like fish, snakes, and many lizards that lack eyelids, their eyes remain open. Despite this anatomical difference, these animals still enter genuine sleep states where their brain activity slows and responsiveness to external stimuli decreases.
Survival Advantages of This Behavior
The ability to sleep with eyes open provides significant survival benefits, particularly for animals in environments with constant threats. Predator vigilance is a primary advantage, as maintaining an open eye allows for quick detection of approaching dangers. Birds, for instance, can adjust the amount of open-eyed sleep based on perceived risk.
Environmental monitoring is another key benefit, especially for aquatic animals. Dolphins and whales, for example, must remain partially conscious to surface for breathing. Open-eyed sleep facilitates this constant awareness. For birds, this adaptation also supports physical safety, enabling them to maintain aerodynamic control during flight or secure perching.
This unique sleep strategy also contributes to physical safety by allowing animals to maintain posture or movement. Walruses can inflate air pouches to stay vertical in water. The ability to rest one half of the brain while the other remains active allows for continuous movement or stability, preventing drowning for aquatic species or falls for perching birds.