Sharks are some of the most incredible creatures on Earth. As apex predators, they play a vital role in maintaining balance in the oceans’ ecosystems. Their biology and behavior have fascinated humans for centuries. One question many people have is whether sharks actually sleep. In this article, we’ll dive into the details and explain what science has revealed about sharks and sleep.
The Debate Around Shark Sleep
There has been considerable debate around whether sharks do in fact sleep. Sharks, together with bony fish and lampreys, belong to a group of aquatic animals known as fish. Most types of fish exhibit periods of rest that appear similar to sleeping. However, sharks’ need to keep water moving over their gills makes their form of rest unique.
For a long time, experts believed sharks had to swim constantly to breathe and could therefore never fully sleep. Their idea was that if sharks stopped moving forward, they would suffocate from lack of oxygen. So how could an animal that never stops swimming truly rest?
Recent research has challenged this notion. Scientists have discovered behavioral, anatomical, and physiological evidence that indicates sharks do enter states of rest and sleep-like states, even while swimming.
However, their form of sleep likely looks quite different from human sleep or the sleep patterns of terrestrial mammals. Sharks may have periods of deep rest as well as lighter rest similar to napping. It’s an area of ongoing research to fully understand how and when sharks rest.
Shark Sleep Patterns
Sharks display various behavioral patterns that suggest they experience some form of sleep:
- Extended periods of stillness or lying motionless on the sea floor
- Decreased responsiveness to external stimuli
- Lowered rates of activity and metabolism
- Hanging in caves or under ledges with minimal movement
- Curling their tails and bodies into a resting posture
These quiescent behaviors can last for several hours at a time. They are observed most frequently at night, indicating sharks are likely programmed to rest and sleep more during this period.
Some species, like nurse sharks and wobbegongs, can pump water over their gills without actively swimming forward. This allows them to fully stop and rest on the ocean bottom.
Other sharks have evolved the ability to sleep while slowly swimming, keeping just enough movement to continue passing water over their gills. Certain adaptations in their anatomy and physiology enable this, which we’ll explore more below.
Half-Brain Sleeping
One of the most fascinating shark adaptations that enables restful sleep is known as “half-brain sleeping.”
Some sharks are able to put one hemisphere of their brains fully to sleep while the opposite hemisphere remains alert. This allows them to maintain awareness of their surroundings for potential threats while still getting rest.
Scientists have found that nurse sharks alternate which half of their brain sleeps and which half remains vigilant. One hemisphere monitors sensory input and motor function while the opposite hemisphere experiences decreased neural activity consistent with deep sleep.
The hemisphere that is sleeping shows characteristic slow-wave brain patterns similar to other sleeping vertebrates. After a period of a few hours, the sharks then swap which brain hemisphere is asleep.
This ability to sleep one half of the brain at a time may enable sharks to achieve continuous rest on a regular basis.
Shark Sleep Postures
When sharks do settle into prolonged resting states, they demonstrate some typical sleep-associated postures:
Curling Up
Some species ball themselves up by curling their bodies and tails into a circular, nose-to-tail position. This posture may help sharks conserve heat and energy when resting. Species like white-tip reef sharks, nurse sharks, and wobbegongs have been observed sleeping in this curled up pose.
Hanging Vertically
Other sharks rest vertically in the water column with their tails curled underneath them. Spurdog sharks are known to hang together in small groups close to the seafloor when sleeping. Pelagic sharks like blue sharks may also hang nearly motionless in deeper waters while sleeping.
Lying on the Seafloor
Sharks that can actively pump water over their gills like nurse sharks, wobbegongs, and epaulette sharks frequently settle down to rest flat on the seabed. They remain still with minimal fin movements for multiple hours.
Wobbegongs even have specially adapted suction-cup-like lobes on their undersides that help them grip onto cave floors or coral reefs while sleeping.
Sleep-Swimming Sharks
The need to keep water flowing over their gills means most open ocean sharks cannot stop swimming altogether for sleep. However, they can enter a unique sleep-like state while slowly swimming forward.
Several adaptations allow sharks to sleep swim:
Unidirectional Water Flow
Sharks have organs called spiracles that force water over their gills in one direction only. Special muscles block the exits, so water flows continually over the gill filaments. This allows oxygen exchange even at low swimming speeds.
Buoyancy Control
Sharks’ lipid-filled livers make them naturally buoyant. Adjusting the amount of oil in their livers allows sharks to fine tune their buoyancy. They can set just the right weight to sleep-swim slowly but steadily without sinking.
Autonomic Swimming
Specialized areas of sharks’ spinal cords contain oscillating networks of nerve cells. These spinal circuits coordinate rhythmic tail movements essential for swimming. They likely take over to maintain a slow but steady swimming motion during restful states.
Reduced Metabolism
Sharks can reduce their metabolism significantly when resting. Their tail oscillators produce just enough movement for ventilation and steering while their overall energy demands decrease.
With these adaptations, fast-swimming sharks like makos and great whites are able to enter restful sleep states even while slowly swimming forward through the open ocean.
Do Sharks Dream?
Given that many sharks clearly enter sleep-like states, could they also experience dreaming like humans do?
Unfortunately, we cannot yet answer this question definitively. Sharks do have the necessary neural anatomy for REM sleep. REM or rapid eye movement sleep is the phase when humans do most of their vivid dreaming.
Some small sharks like Port Jackson sharks and epaulette sharks have been observed experiencing REM-like states with rapid eye and fin movements. Larger sharks also likely enter REM sleep but direct observations are difficult.
Until we invent shark dream-reading technology, whether sharks actually dream remains a mystery. But it’s clear their brains do undergo similar sleep cycles and patterns to many other vertebrates. Dreaming does seem well within the realm of possibility for these evolutionary advanced fish.
Shark Sleep Habits by Species
Different shark species have varying sleep habits and abilities:
Nurse Shark
Nurse sharks are one of the most frequent and prolonged sleepers. They can pump water over their gills with their muscular pharynx. This enables them to completely stop swimming and settle on the sea floor for many hours.
Wobbegong
Wobbegongs also spend a large portion of the day resting stationary on the ocean bottom. They use their specially adapted ventral suction lobes to wedge themselves into crevices and under ledges to sleep.
Epaulette Shark
These small sharks live on shallow reef flats that become exposed at low tide. Epaulettes can survive for hours out of water by entering a stupor state. They may also exhibit this extended resting behavior when submerged.
Whitetip Reef Shark
Whitetips frequently shelter in caves during the daytime, where they remain motionless in small groups for hours. At night, they emerge to actively hunt for prey.
Blue Shark
By adjusting their liver oils, blue sharks can hover nearly motionless in deep water columns. They likely enter prolonged sleep-swimming states in these open ocean habitats.
Great White Shark
Despite their constant need to swim, great whites are known to exhibit long periods of quiet restfulness. Their slow sleep-swimming may happen in deep offshore areas.
Lemon Shark
Tracking studies reveal lemon sharks sleep for at least 8 hours daily. They rest motionless in caves and crevices for long periods before emerging at dawn to actively hunt.
| Species | Sleep Habits |
|---|---|
| Nurse Shark | Rests on sea floor, pumps water over gills |
| Wobbegong | Wedges into crevices to sleep stationary |
| Epaulette Shark | Enters stupor state out of water and when submerged |
| Whitetip Reef Shark | Sleeps motionless in caves during daytime |
| Blue Shark | Sleep-swims slowly in deep offshore waters |
| Great White Shark | Exhibits long periods of quiet restfulness while swimming |
| Lemon Shark | Sleeps for 8+ hours motionless in caves before dawn hunting |
This table summarizes the sleep habits of some known shark species that demonstrate distinct resting behaviors. The differences reflect adaptations in anatomy, habitat, and lifestyle across these aquatic predators.
Shark Brainwave Patterns During Sleep
Scientists have measured sharks’ brainwave activity to gain insights into their sleep states and cycles.
Electroencephalogram (EEG) readings of a sleeping shark’s brain reveal:
- Slow-wave activity indicating deep, restorative sleep
- Periods of rapid eye movement (REM) similar to other vertebrates
- Brain hemispheres showing asynchronous activity, suggesting half-brain sleep
- Shifting progression between different sleep stages
Port Jackson sharks demonstrate clear REM sleep with active rapid eye movements. Larger shark species are also likely to experience REM but direct observation of eye movements is difficult.
Overall, sharks’ brainwave patterns during rest resemble those of terrestrial vertebrates in REM and non-REM sleep. Their brain cycling indicates they undergo a form of true sleep, even if slight swimming motions persist.
How Long Do Sharks Sleep Each Day?
Sharks likely need to sleep for a substantial portion of the day, similar to most other animals. However, exact sleep requirements may vary between different species.
Captive sharks have been recorded sleeping for 10-16 hours per 24-hour period. Some key points about their daily sleep needs:
- Larger shark species may only sleep in 1-2 hour blocks distributed over the day.
- Smaller sharks can sleep continuously for longer periods of 8-12 hours.
- Sharks are mostly nocturnal, so daylight hours are their primary sleep period.
- Juvenile sharks need more sleep than adults, sometimes over 16 hours a day.
In the wild, sharks likely get most of their sleep done during daytime sheltering periods. For example, lemon sharks remain motionless in their sheltered caves for an estimated 8 hours daily before nighttime hunting.
More research is still needed to determine average sleep times across different ages, sizes, and species in natural conditions. But available evidence indicates most sharks need a good 6-12 hours of quality rest per 24-hour cycle.
Why Do Sharks Need To Sleep?
Sleep serves crucial biological functions for sharks, just like other animals:
Energy Conservation
Sleep allows sharks to operate with reduced metabolism, conserving valuable energy. Their slowed activity and lowered body temperature during rest periodssave calories.
Tissue Growth & Repair
Deep, restorative sleep enables cell repair and tissue growth. Sharks heal injuries, build muscles, and renew epithelial cells during rest. Sleep deprivation hinders these processes.
Neural Restoration
Sleep allows sharks’ brains to recharge neural batteries, clear waste metabolites, and strengthen connections. Without enough rest, their brains cannot function properly.
Memory Consolidation
Sharks likely consolidate learning and memories during certain sleep phases. Non-REM and REM sleep play key roles in memory formation in other animals.
Immune Support
Sleep strengthens immune function in vertebrates. Shark sleep helps optimize lymphocyte production, antibody reactions, and other defenses against disease.
In summary, sleep provides sharks with the same regenerative benefits it offers to fish, birds, and mammals. Failing to get adequate rest would seriously impair their health, growth, brain function, and survival.
Threats to Shark Sleep
Although sleep is essential, some modern human activities may be interfering with sharks’ ability to get enough rest:
Noise Pollution
Boat traffic, sonar devices, and seabed drilling create loud noise pollution in ocean habitats. This acoustic disturbance could awaken sleeping sharks and curtail sleep periods.
Artificial Lighting
Nighttime illumination from ships, platforms, and coastal buildings may inhibit sheltering and sleep in shallow-water sharks. Light can disrupt their natural rhythms.
Habitat Disruption
Loss of sheltered caves and crevices through dredging, fishing, and coastal development removes crucial daytime sleeping sites for many sharks.
Climate Change
Rising ocean temperatures likely influence the timing, duration, and quality of shark sleep. Warmer waters could impair restful states.
More research is needed to determine precisely how these human pressures impact different shark species’ ability to sleep and maintain healthy rest cycles. Addressing threats to sharks’ essential sleep could be important for their conservation.
Conclusion
In conclusion, scientific evidence clearly shows sharks do experience true sleep states, even while gently swimming. Behaviors like hanging motionless or lying on the bottom, reduced energy use, brainwave patterns, and optimal rest times all indicate sharks sleep much as other vertebrates do.
Different species have evolved adaptations allowing them to rest deeply while still ventilating their gills. Sharks appear to benefit from sleep in the same regenerative ways as land animals. Yet modern disturbances may interfere with sharks’ ability to get enough sleep. Understanding and preserving sharks’ natural rest habits will be key for their wellbeing and survival into the future.