Understanding Shark Rest
The idea that sharks never stop swimming or sleeping is a common misconception. While sharks do not experience sleep in the same way humans or other mammals do, they certainly have periods of rest. This rest is characterized by reduced activity and a decrease in responsiveness to their environment, rather than a deep state of unconsciousness.
Scientists observe shark rest through indicators such as immobility, relaxed postures, and diminished reactions to external stimuli. A key finding in recent studies is that sharks in a resting state show a lower metabolic rate, indicating energy conservation, which aligns with a definition of sleep across many species.
Different Ways Sharks Rest
The method a shark uses to rest depends on its respiratory system. Sharks breathe by passing oxygen-rich seawater over their gills, but they achieve this in two primary ways. This difference dictates whether they can remain stationary or must keep moving even while resting.
One group consists of obligate ram ventilators, such as great white sharks, mako sharks, and whale sharks. These sharks must constantly swim forward to force water over their gills, a process known as ram ventilation. If they stop moving, they risk suffocating.
To rest, these sharks engage in “active rest” by swimming slowly or utilizing ocean currents. They can enter a trance-like state, maintaining minimal movement to ensure water flow over their gills while still getting a form of rest. For example, a great white shark was observed drifting in shallow water against a strong current with its mouth open, appearing almost catatonic.
Conversely, buccal pumpers can actively pump water over their gills using cheek and neck muscles, allowing them to breathe while stationary. This group includes species like nurse sharks, lemon sharks, and carpet sharks. These sharks can rest motionless on the seabed or within sheltered areas. Some, like nurse sharks, can even partially bury themselves in the sand while still breathing through this method or by using spiracles, small openings behind their eyes that draw in water. Recent research has even shown that some sharks previously thought to be obligate ram ventilators, like the grey reef shark, might be capable of switching to buccal pumping to rest.
What Happens in a Shark’s Brain
While sharks do not exhibit the same brainwave patterns as mammals during sleep, their brains undergo changes during resting periods. Scientists use behavioral cues and physiological measurements, such as metabolic rate, to infer these states. During rest, sharks show reduced responsiveness to stimuli, suggesting an altered brain state that is less aware than full wakefulness.
Some hypotheses suggest that certain shark species, particularly obligate ram ventilators, might engage in a form of unihemispheric slow-wave sleep. This unique adaptation, observed in dolphins and whales, allows one half of the brain to rest while the other remains active. This mechanism would enable them to continue swimming and remain aware of their surroundings for survival, even while resting. Although direct electrophysiological studies on shark brains during rest are limited, the observed reduction in metabolic activity points to a restorative process.
Common Resting Spots
Sharks choose their resting locations based on their breathing method and natural habitats. Obligate ram ventilators, which must keep moving, often utilize steady ocean currents for their active rest. They might be observed in open waters, slowly cruising or finding small gullies where currents can passively ventilate their gills.
Buccal pumping sharks, capable of remaining stationary, seek out sheltered environments for their rest. Nurse sharks, for example, are frequently found resting on sandy seabeds, under ledges, or within caves and crevices during daylight hours. They are nocturnal, becoming more active at night for hunting, and often return to the same preferred resting spots day after day. These species may even rest in groups, sometimes piling together in these secure locations.