Sharks do not sleep in the same way humans or other mammals do, making the question of their rest complex. Instead, they engage in periods of rest, a state involving reduced activity and responsiveness that serves a similar restorative purpose to sleep. Understanding shark rest requires exploring their unique biology and observable behaviors.
Breathing and Resting Strategies
A shark’s ability to rest depends on its method of breathing. Sharks primarily use two mechanisms to move oxygenated water over their gills: ram ventilation and buccal pumping. Ram ventilation requires continuous forward movement to force water through the mouth and over the gills. Species like great white, mako, and hammerhead sharks use this method, risking suffocation if they stop swimming.
In contrast, many bottom-dwelling sharks, such as nurse, lemon, and Port Jackson sharks, employ buccal pumping. This method involves actively pumping water over their gills using muscles in their mouth and pharynx, allowing them to remain stationary. Some sharks that use buccal pumping also possess spiracles, small openings behind their eyes, which assist in drawing water over the gills when resting on the seafloor. Certain species, like the sand tiger shark and grey reef shark, can switch between both methods.
Observing Shark Rest
Resting behaviors vary among shark species, reflecting their diverse breathing strategies. Sharks relying on buccal pumping are often seen motionless on the seafloor or within caves. Nurse sharks, for instance, frequently spend their days inactive, sometimes piling together in groups. During these periods, they show reduced responsiveness to external stimuli.
Sharks using ram ventilation must keep moving to breathe, so their resting involves a different approach. Great white sharks have been filmed swimming slowly at night, maintaining a gentle speed of about 3 kilometers per hour, with mouths open to allow water to flow freely over their gills. This behavior suggests they conserve energy while ensuring oxygen supply. Draughtsboard sharks, when resting, adopt a flat, rigid body posture and may keep their eyes open, though eye closure is more common during daytime rest.
Understanding Shark Brain Activity
Defining sleep in sharks scientifically goes beyond mere observation, posing challenges for researchers. Sharks do not exhibit the same distinct brainwave patterns as mammals, such as REM sleep. However, studies indicate they enter periods of reduced responsiveness and lower metabolic rates, consistent with a resting state.
Research on draughtsboard sharks has shown that after five minutes of inactivity, they display reduced sensitivity to electrical stimulation and decreased oxygen consumption, suggesting they are in a sleep-like state. While electroencephalography (EEG) studies on sharks face challenges due to signal instability, changes in muscle tone have been noted, with the lowest muscle tone observed during periods of rest. This indicates that while sharks may not become unconscious, their brains experience altered activity. The ability of some sharks to continue swimming while resting parts of their brain, possibly due to spinal cord control, points to a unique evolutionary adaptation for rest.