Do Sharks Sleep?02:43

Sharks are amazing creatures. Dubbed as kings of the sea, they are one of the oldest living creatures on earth, having been alive even before the dinosaurs. While they’ve been around for millions of years, it’s quite surprising to know that one aspect of their biology is actually quite mysterious. For many years, scientists have asked the question, “Do sharks sleep?”

It is well-known that sharks need to move or swim all the time to be able to breathe. They need to keep moving to allow oxygen to enter through their mouths and exit through their gills. That would mean sharks would need to swim all the time to stay alive. But scientists have actually discovered a few explanations as to how sharks don’t necessarily have to move all the time to be able to breathe, while others believe a unique mechanism allows them to “sleep,” while swimming at the same time.

How sharks sleep is an extraordinary topic that’s not so easy to explain or understand. In fact, only a handful of species have been studied with regard to their sleeping patterns.

Nevertheless, there are many things humans can learn from sharks, with their sleeping habits being one of them. Let’s dig deeper into the world of sharks and how they sleep.

How Sharks Breathe and Its Connection to Sleeping

Do sharks sleep? The answer to this is not so simple. What humans know about sleep is being unconscious and immobile, being active during the day and sleeping at night, and separating time between being awake and being asleep.

It has long been speculated that sharks don’t sleep because they need to swim all the time to be able to breathe. But recent discoveries have debunked this myth by exploring how sharks breathe.

The main topic with sharks and sleeping is their breathing ability. Sharks need to move to be able to breathe through their gills. This process is known as Ram Ventilation, wherein they draw water through their mouth and force it out through their gills. They are only able to do this when they move forward, allowing the water to go through their bodies.

However, not all sharks need to move all the time to be able to breathe. Some sharks can breathe even when they’re immobile. They are able to do this by using their spiracles that force water out of their gills. Spiracles are similar to nostrils, which are openings just behind the eyes of some sharks, something similar to rays. These spiracles allow sharks to breathe even when they’re lying on the sea bottom or buried under the sand.

It might be safe to assume that sharks that are able to breathe through swimming don’t sleep; while sharks that can breathe without moving do sleep. The truth to this answer is that both kinds of sharks do sleep, and it could be due to the way they swim.

Scientists discovered that the area of the shark’s body that controls how they swim is not located in their brain, but in their spinal cord. This could explain how a shark can sleep while swimming at the same time. With this mechanism, sharks can sleep while also being able to breathe.

Start tracking your daily Calorie Burn and Sleep Habits with an Apple Watch or Fitbit

When and Where do Sharks Sleep?

Sharks may sleep in the deeper parts of the ocean, but some studies have also shown they can sleep in shallow waters. When they’re in the deeper parts of the ocean, they sleep in coral reefs or on the seabed where they bury themselves in the sand. Some sharks also roam around coral reefs as they’re sleeping.

Unlike humans where we usually sleep at nighttime, there is no separation of time for sharks when it comes to sleeping. This is because all throughout the day, the shark goes through phases of wakeful and restful periods.

It’s also important to note that the human concept of sleep does not apply to sharks. Sharks are not completely unconscious when they sleep. In fact, only half of their brains are unconscious while the other half is active. Sleep for sharks is more like a resting period rather than deep sleep where they become immobile.

How Long Do Sharks Sleep?

It has been speculated that sharks don’t sleep for more than a few minutes but studies have shown that they can actually sleep for hours. However, as mentioned earlier, their days oscillate between waking and sleeping, so it can be hard to tell how many hours they do sleep in a 24-hour period.

Researchers have revealed that a shark’s rest cycle is dependent on its Circadian Rhythm. Sharks rest or sleep based on their instincts, and not according to a specific schedule or when they’re tired. If their Circadian Rhythm tells them it’s time to sleep, they sleep.

For humans, we sleep in relative to our time spent active. When we are active during the day, we sleep at night; when we are active at night, we sleep during the day. This does not apply to sharks.

This information was revealed in a study done by Micheal L. Kelly and others and published in the Journal of Biological Rhythms, dated June 11, 2020. If humans were to learn from this, following our Circadian Rhythm to our sleeping schedule allows us to have a healthy sleeping pattern, because it follows a natural cycle that our bodies are meant to follow.

The Circadian Rhythm is the internal clock of the body that sends us signals when we eat, when we wake, when we sleep, and all our biological schedules. When we deviate from our Circadian Rhythm, we feel tired, fatigued, and have struggles sleeping. We then need time to adjust to a new schedule or would need to get back to our original schedule for a healthy routine.

Do Sharks Close Their Eyes When They Sleep?

Sharks don’t close their eyes when they sleep because they don’t have eyelids. When they bite their prey, a translucent “nictating membrane” protects their eyeballs. For Great White Sharks, they protect their eyes before taking a bite of their prey by rolling their eyeballs.

A shark may be sleeping and immobile but its eyes will still be open. As mentioned earlier, only half of the shark’s brain is active when they’re asleep. Divers may notice that although a shark may be asleep and immobile, its eyes will still follow them around. A shark’s pair of eyes will continue to track divers swimming around them even when they’re asleep.

Do Sharks Sleep Upside Down?

No, sharks do not sleep upside down. However, they do go into a state of tonic immobility when they are turned upside down. There have been instances when killer whales purposely turn sharks upside down in order to bring them into this state to be able to suffocate and prey on them.

Tonic immobility is a state where a shark is paralyzed for about 15 minutes before it becomes mobile again. This state can also be induced by placing hands on the sides of the shark’s snout near the general area around the eyes. Sharks’ muscles contract and their breathing slow down during this state.

Can A Shark Drown While Sleeping?

Yes, a shark can drown while sleeping if it’s an obligate ram ventilator and it stops moving, or if it gets caught in a fishing net and they experience hypoxia or lack of oxygen.

There are over 400 species of sharks and about 2 dozens of them are obligate ram ventilators, which are sharks that need to swim all the time to be able to breathe. The rest of the species of sharks can sleep without swimming.

These obligate ram ventilators are at risk of drowning when they sleep. Among these sharks are the Great White Shark, Hammerhead Shark, Whale Sharl, Mako Shark, Megamouth, and Thresher Shark among others.

Defending Against Preadotrs During Sleep?

Sharks are considered to be the top marine predators, especially the Great White Shark. However, recent discoveries have revealed that they are actually preyed upon by Orcas or Killer whales.

While sharks are generally not threatened by predators, the way they sleep has actually been credited for their longevity as a species.

When a shark is sleeping, both of its eyes are wide open and half of its brain is active. Because of this, they are perfectly suited for survival. Carbon dating of shark teeth that were excavated proved that sharks swam the seas of the Earth long before dinosaurs.

They are considered to be the fastest, most formidable predators in the water and credited for regulating the marine food chain. Their sleeping pattern has greatly contributed to their longevity in the marine kingdom since even in deep sleep states, they remain partially conscious and active.

How to Fake Sleep

Do Great White Sharks Sleep At All?

Being obligate ram ventilators, it has been belied that the Great White Shark does not sleep, or that it continues to swim while resting or asleep. However, a video from Discovery Channel shows a 2016 footage wherein a female white shark seems to be sleeping and immobile while in shallow waters.

The female Great White Shark swam at night around the Guadalupe Island near the Baja California peninsula in Mexico. It is believed that the footage is the very first time a Great White Shark has been captured on film while in a sleep-like state. Though it continued to swim, it appeared the shark was in a catatonic state, as if it were hypnotized.

It swam in shallow waters, facing the oxygen-rich strong current, with its mouth wide open. Researchers believed that the shark did this to allow the current to continuously bring water into its mouth and flood into its gills, allowing it to breathe while sleeping.

How Do Other Species Of Sharks Sleep?

Ceratin species of sharks have different sleeping or resting habits, which are just as interesting as the Great White. Here are some of them:

• Greenland Shark

The Greenland Shark is one of the oldest known sharks in the world, where they are known to live from 200 to 500 years old. They are also one of the most mysterious and less studied due to their preferred habitat, which is in the North Atlantic Ocean and the Arctic Ocean. They prefer to live in water that’s between -1 to 10 degrees Celsius.

They are also known as sleeper sharks, not because they sleep a lot, but because they move slowly. Their swimming speed is about 1km per hour. Surprisingly, it has been revealed that they are obligate ram ventilators, which is a feature typical of fast-moving sharks. They need to move in order to breathe as they sleep.

• Whale Sharks

Whale sharks are considered to be the largest fish species in the world, known to live between 70 to 100 years old. They can weigh an average of 20 tons and their size is relative to a bus.

They live in warm temperate seas, and are filter feeders, living on sea planktons. Their sleeping or resting habit is known as Yo-Yo Swimming, a habit where they go through brief periods of rest while they descend to the bottom of the ocean. Whale sharks travel for great distances when they migrate and use this kind of swimming technique to rest. They swim to the surface then rest or sleep as they descend into the deeper part of the ocean.

• School Sharks

School sharks are found in temperate waters of the Northern Atlantic and the Mediterranean Sea. They can live up to 55 years and tend to hang out around the coast, making them a pretty common sight. They are obligate ram ventilators and need to swim to be able to breathe when they sleep.

• Tiger Sharks

After the Greate White, Tiger sharks are the most feared sharks in the world. They can live between 27 to 50 years old, and live in tropical or subtropical waters. Just like the Great White, they are obligate ram ventilators that need to keep swimming when they sleep.

• Hammerhead Sharks

Hammerhead sharks are aggressive hunters and can live to about 30 years. They are known for their flattened and extended heads and reach up to 13 to 20 feet in size. They are also obligate ram ventilators which need to move to be able to breathe when they sleep.

Conclusion

Sharks are amazing creatures of the water. While there is still so much to learn and discover about their sleeping habits, it’s clear to see that even though some shark species need to swim to be able to breathe, their bodies still need sleep to be able to survive.

The concept of sleep for humans is quite different from how sharks sleep, but one thing that humans can learn from these creatures is how they listen to their bodily instinct to rest. While humans have their Circadian Rhythm to thank to indicate when it’s time to sleep, humans don’t always follow this internal clock. Because of this, humans suffer from sleep deprivation, insomnia, and other sleep issues that can lead to serious problems. Maybe if we just listened to our Circadian Rhythms, we would enjoy better longevity and survival, just like sharks.

While humans may have problems with sleeping due to an offset Circadian Rhythm, there are many things that we can do to help us improve our habits. For starters, sleep trackers can help us monitor our sleeping behaviors, allowing us to make proper changes that could eventually help us create better and healthier sleeping habits.

Check out the Fitbit Charge 4 Sleep Tracker!