Lying down for a good night of sleep and dreams that transport us up and away from the worries of life is something most of us take for granted. When you see sharks lying motionless beneath a sheltered reef structure or down in the sand next to a wreck, you might think he’s enjoying toothsome dreams of his own; but sharks and other marine animals don’t sleep the same way we do. How do they sleep while maintaining the movement that keeps water running across their gills so that they may breathe? Let’s investigate.
The Science of Shark Sleep
Some sharks do need to keep up a little movement to breathe while resting, while others can relax completely, thanks to little organs called spiracles, which are located behind the sharks’ eyes. Water enters these organs, forcing water across the shark’s gills, enabling it to lie motionless.
Sharks that rely on ram ventilation must keep their mouths open while moving forward to breathe; these sharks keep moving while resting at least part of their brains, engaging in an activity called sleep swimming. In this state, they move in a way which indicates that they are only partially aware of their surroundings, much in the same way that people who sleepwalk are capable of movement while completely asleep.
Scientists have yet to determine exactly why it is that we and other living things require sleep; what they do know is that when a state called “Torpor” occurs, brain wave patterns change dramatically. From experiments that were carried out on Spiny Dogfish, a small shark species found throughout temperate coastal regions worldwide, scientists have discovered that the Central Pattern Generator, an organ that coordinates swimming movements, is located in the sharks’ spinal cord rather than in its brain, meaning that it is possible for a shark to swim while experiencing a state of torpor. Even more fascinating is the possibility that some sharks shut down small portions of their brains sequentially, allowing for a semi-alert state that would explain why sharks that appear to be sleeping motionless, such as those found at sites like Sleeping Shark Cave at Isla Mujeres in Cancun, do track divers with their eyes.
While we humans are diurnal, and while most dives take place during daylight hours, many shark species are much more active at night, as well as at dawn and dusk, than they are during the day. Sonic telemetry studies focusing on nighttime reef activities have shown that white tips and tiger sharks are extremely active at night. Great White Sharks, too, have been found to hunt more at night than during the day; telemetry pods placed along the reef bottom around the Farrallon Islands have shown that these giants feed opportunistically on sleeping bottomfish.
A study of lemon sharks showed that it was more difficult for them to stay motionless than it was to keep up enough motion to breathe well; these sharks, like many others, seem to position themselves so that current pushes water into their mouths while they are resting, so they can enter a more relaxed state.
Next time you see a shark at rest, take a moment to notice how the animal has positioned itself, and watch for small swimming movements. Remember, sharks can wake up in a flash; always keep a safe distance and never harass a shark.