Sharks, ancient predators of the ocean, often spark curiosity regarding their behaviors, especially how they manage to rest. A common misconception suggests these marine animals never truly sleep, primarily because many must keep moving to breathe. This unique aspect of their biology leads many to wonder about their periods of inactivity and what “sleep” means for a creature constantly navigating the vast underwater world. Understanding their resting patterns requires looking beyond human-centric definitions of sleep. It involves exploring the unique adaptations that allow these diverse species to recover and conserve energy in their dynamic environments.
Do Sharks Truly Sleep?
While sharks do not experience sleep in the same manner as humans, they certainly enter states of rest or reduced activity. Sleep, in a broader biological sense, is characterized by prolonged inactivity, relaxed postures, and reduced responsiveness to the environment. For sharks, this translates into observable behavioral and physiological changes. Recent studies provide insights into these resting states, suggesting they are a form of sleep. For example, Port Jackson and draughtsboard sharks show reduced responsiveness during periods of inactivity lasting more than five minutes, aligning with scientific definitions of sleep.
These sharks sometimes close their eyes, or keep them open, and may adopt a flattened posture close to the ground. This suggests energy conservation, a common purpose of sleep across the animal kingdom. Some evidence also suggests a phenomenon called unihemispheric slow-wave sleep (USWS) in certain shark species, where one half of the brain rests while the other remains active. This adaptation allows them to remain aware of their surroundings and potentially continue essential functions like swimming.
How Different Shark Species Rest
The way different shark species achieve rest is largely determined by their respiratory methods, primarily ram ventilation and buccal pumping. Sharks extract oxygen from water passing over their gills, but the mechanism for this water flow varies significantly among species. This physiological difference dictates whether a shark can remain stationary while resting or must continue moving.
Ram ventilation requires a shark to swim continuously with its mouth open, forcing water over its gills to obtain oxygen. Sharks like great whites, mako sharks, and hammerheads are obligate ram ventilators, meaning they must maintain constant forward motion to breathe. If they cease movement, water flow over their gills stops, preventing oxygen uptake. Observations suggest that some ram ventilating sharks might engage in “yo-yo swimming” or “sleep swimming” with reduced activity, allowing for rest while still moving.
In contrast, other shark species utilize buccal pumping, which allows them to actively pump water over their gills without continuous movement. These sharks use muscles in their cheeks and throat to draw water into their mouths and push it out through their gill slits. This method enables them to rest motionless on the seafloor or within shelters. Nurse sharks, wobbegongs, and lemon sharks are prime examples of species that employ buccal pumping and are frequently observed resting stationary in caves or on sandy bottoms. Some species, such as the bull shark or grey reef shark, can switch between both methods depending on their activity level.
The Science of Shark Rest
Scientific understanding of shark rest primarily comes from observing behavioral changes and, more recently, through physiological measurements. During periods of rest, sharks often exhibit reduced responsiveness to external stimuli, a key indicator of a sleep-like state. For instance, nurse sharks in captivity show decreased reactions to divers when at rest, and draughtsboard sharks require stronger electrical stimulation to react after resting for more than five minutes.
Researchers also note altered swimming patterns, such as reduced speed or adopting a rigid, flattened body posture on the seafloor. While direct electrophysiological studies of brain activity in wild sharks are challenging, some research indicates changes in metabolic rates during these periods of inactivity. A 2022 study on draughtsboard sharks found lower oxygen intake during rest, providing physiological evidence consistent with sleep. These observations suggest that even without the distinct brainwave patterns seen in mammals, sharks undergo a restorative process similar to sleep, crucial for their overall health and energy conservation.