When observing sharks, a common question arises regarding their resting behaviors: do they close their eyes? Unlike many animals, sharks exhibit unique patterns of rest that differ significantly from human sleep.
Understanding Shark Rest
Sharks do not experience sleep in the same way humans or other mammals do, characterized by a deep unconscious state. Instead, their “sleep” involves periods of reduced activity and lowered responsiveness to their surroundings. During these times, sharks often settle on the seafloor or remain motionless in the water column, conserving energy. Studies on species like the draughtsboard shark have shown a decrease in metabolic rates during these restful periods, indicating a genuine state of rest.
Some shark species, such as the nurse shark, lemon shark, and whitetip reef shark, have been observed lying still on the seafloor or gathering in caves. This reduced activity allows them to conserve energy. Brain activity research suggests that some sharks might engage in a form of unihemispheric slow-wave sleep. This adaptation permits one half of their brain to rest while the other remains partially active, allowing them to maintain some environmental awareness.
How Sharks Breathe While Resting
A shark’s ability to rest, and how it does so, is closely tied to its method of respiration. Sharks breathe by passing oxygen-rich water over their gills. There are two primary mechanisms for this: ram ventilation and buccal pumping. Ram ventilators, such as the great white shark, mako shark, and many hammerhead species, must continuously swim forward to force water over their gills. If these sharks stop moving, they risk suffocating as insufficient water flows over their gills.
Conversely, buccal pumpers can actively draw water into their mouths and pump it over their gills using specialized muscles. This method allows species like the nurse shark, wobbegong, and lemon shark to remain stationary, often resting on the ocean floor, while still breathing effectively. Some sharks possess spiracles, small openings behind their eyes, which also assist in drawing water over the gills, particularly when resting on the bottom or buried in sand. Certain species, like the sand tiger shark, can even switch between ram ventilation and buccal pumping depending on their activity level.
Why Their Eyes Stay Open
The observation that sharks often rest with their eyes open stems from their unique ocular anatomy. Most shark species do not possess movable eyelids that can fully close over the eye in the way human eyelids do. Their eyelids, when present, are primarily for protection rather than for blinking or moisture.
Many sharks have a nictitating membrane, a translucent or opaque protective layer. This membrane slides horizontally across the eye to shield it from injury. However, this is not a mechanism for “closing” the eye during rest. Even in a resting state, a shark’s eyes typically remain open, allowing them to maintain environmental awareness. While some studies note occasional eye closure in species like the draughtsboard shark, this behavior is often influenced by external factors like light, not sleep.