Do Lizards Sleep With Their Eyes Open?

Sleep patterns vary widely across the animal kingdom, leading to questions about how specific creatures like lizards rest. Understanding how these reptiles rest involves exploring the physical structures of their eyes and their unique behavioral adaptations for survival.

A Look at Lizard Eyelids

The anatomy of a lizard’s eye is a primary factor in how it appears while sleeping. Most lizard species, including iguanas and bearded dragons, have upper and lower eyelids that function similarly to those of humans, allowing them to blink and close their eyes. These eyelids protect the eye’s surface from debris, help distribute moisture, and block out light during rest. Many lizards also possess a nictitating membrane, or “third eyelid,” a translucent tissue that moves horizontally across the eye for extra protection without completely blocking vision.

However, not all lizards have movable eyelids. Many species of gecko and some skinks have a fixed, transparent scale called a spectacle that permanently covers each eye. This fused structure protects the eye but means these lizards are physically unable to close their eyes. To keep the spectacles clean and moist, these lizards, such as the crested gecko, use their tongues to wipe the surface.

Lizard Sleep Behavior: Eyes Open or Shut?

A lizard’s sleep behavior is closely tied to its anatomy. Species that possess eyelids commonly sleep with their eyes fully closed, often observed in a prone body posture indicating a state of rest. For lizards that lack eyelids, such as many geckos, they must sleep with their eyes open as they have no other choice.

Some lizards exhibit asynchronous eye closure, where one eye remains open while the other is closed. This is a sign of unihemispheric slow-wave sleep, a state where one half of the brain rests while the other half remains alert. This adaptation is useful for predator vigilance, allowing the lizard to monitor its surroundings for threats even while sleeping. Studies show that when a potential predator is introduced, lizards increase this behavior and orient their open eye toward the threat.

Identifying When a Lizard is Asleep

Since the state of a lizard’s eyes is not always a reliable indicator of sleep, other behavioral cues can provide confirmation. One of the most apparent signs is a significant reduction in movement. A sleeping lizard will remain still for extended periods, often in a relaxed posture, such as lying flat or draped over a branch. Depending on the species, some may even adopt specific sleeping positions, like tucking their head for security.

Observing the lizard’s breathing pattern can also be informative. When asleep, their breathing becomes slower, shallower, and more regular. A sleeping lizard will also show a reduced responsiveness to minor disturbances, although it can still awaken quickly if it perceives a genuine threat. For some species, like crested geckos, the position of their crests can be a clue; upright crests indicate alertness, while flattened crests often signify sleep.