Sharks' Unique Ability: Staying Awake While Asleep

Sharks are widely believed to be perpetually in motion, leading to the misconception that they do not sleep. However, emerging evidence suggests that sharks do sleep, albeit not in the same way humans do. While some shark species need to keep moving to breathe, others can remain stationary thanks to special structures that force water through their gills. Sharks that can breathe without swimming have exhibited sleep-like behaviour, such as reduced responsiveness and lower metabolic rates. Some sharks even close their eyes during these periods, while others keep them half-open or fully open. The duration of shark sleep also varies, with some species sleeping in short bursts and others remaining in a sleep-like state for much of the day or night.

Some shark species can breathe while stationary

In contrast, some shark species must continuously swim to breathe, a process known as ram ventilation. These species are known as obligate ram ventilators. They force water through their gills by drawing it in through their mouths. Examples of obligate ram ventilators include great white sharks, tiger sharks, and hammerhead sharks.

Some shark species can switch between the two methods of breathing. For example, bull and tiger sharks can use both ram ventilation and buccal pumping. Interestingly, new evidence suggests that grey reef sharks, previously believed to be obligate ram ventilators, can also switch to buccal pumping during periods of inactivity. This discovery challenges our fundamental understanding of shark behaviour and highlights how much we still have to learn about these fascinating creatures.

While it is clear that some shark species can breathe while stationary, the question of whether or not sharks truly sleep is more complex. Sleep is typically defined as a period of motionlessness with reduced environmental awareness. However, sharks that are resting remain vigilant, with their eyes open and pupils monitoring the movement of surrounding creatures. Thus, it is more accurate to describe sharks as engaging in periods of deep rest rather than sleep in the traditional sense.

Sharks may sleep with their eyes open

While sharks do engage in periods of rest, they do not fall asleep in the traditional sense. Their eyes remain open, and they continue to monitor their surroundings.

Sharks that can rest while remaining stationary include the whitetip reef shark, the Caribbean reef shark, the nurse shark, the wobbegong, and the lemon shark. These species possess special structures called spiracles, which force water through their gills, allowing them to breathe while stationary.

Recent research has provided the first physiological evidence of sleep in draughtsboard sharks, a species of bottom-dwelling shark native to New Zealand. Scientists observed that these sharks can sleep with their eyes open or closed, depending on the amount of surrounding light. During the day, they occasionally slept with their eyes shut, but at night, they mostly kept their eyes open. This suggests that their eye closure may be more related to light conditions than the sleep state itself.

The research also found that draughtsboard sharks altered their posture when asleep, flattening and nestling closer to the ground. Additionally, their oxygen consumption dropped during periods of rest, indicating a metabolic change associated with sleep.

While the exact sleep behaviour of sharks remains a mystery for many species, the ability to sleep with their eyes open may be a common trait among certain shark species.

Sharks may sleep using half their brain

While it was previously believed that sharks do not sleep at all, new evidence suggests that some shark species do indeed sleep, although not in the traditional sense.

Sharks that must keep moving to breathe, known as obligate ram ventilators, have shown no evidence of sleep behaviour. However, scientists have hypothesised that these sharks may sleep using only half their brain, similar to whales and dolphins. This theory is supported by the observation that some ram-ventilating shark species, such as the grey reef shark, can switch to buccal pumping behaviour, allowing them to rest while stationary.

Whales and dolphins are known to sleep with one eye open, monitoring their environment with the half of the brain that remains awake. It is possible that obligate ram ventilating sharks have evolved a similar strategy, allowing them to continue moving while sleeping. This hypothesis is yet to be confirmed, and the exact mechanisms by which these sharks sleep remain a mystery.

It is important to note that sleep in sharks may look very different from what we typically associate with sleep in humans or other mammals. Sharks may exhibit reduced responsiveness to stimulation and a lower metabolic rate during sleep, but their eyes often remain open, and they may continue to move or swim.

Further research is needed to fully understand the sleep behaviour of different shark species, including the potential role of half-brain sleep in obligate ram ventilators.

Sharks may sleep facing into ocean currents

While it was long believed that sharks don't sleep at all, recent studies have shown that some species of sharks do sleep, but not in the traditional sense. Sharks that are able to rest while remaining stationary include the whitetip reef shark, the Caribbean reef shark, the nurse shark, the wobbegong, and the lemon shark.

Sharks that need to swim constantly to keep water moving over their gills seem to have active periods and restful periods, rather than undergoing deep sleep as humans do. They seem to be "sleep-swimming", with parts of their brain being less active, or "resting", while the shark continues to swim.

One hypothesis for how obligate ram ventilators sleep is that they sleep facing into ocean currents. This hypothesis is based on the idea that sharks can sleep with their whole brain but require the currents to help them ventilate by running oxygen-rich water over their gills, allowing them to remain motionless.

However, there is currently no convincing evidence that sharks do this. A recent study on grey reef sharks, a species previously thought to be obligate ram ventilators, found that they were able to rest without facing into ocean currents. The sharks were facing in all different directions and there were little to no currents at the sites, indicating that they were not relying on the currents for ventilation.

While the mystery of how obligate ram ventilators sleep remains unsolved, the discovery that grey reef sharks can switch their mode of breathing and rest provides new insights into the sleep behaviour of sharks and challenges our current understanding of these fascinating creatures.

Sharks may sleep in caves or on the ocean floor

While it is a common misconception that sharks do not sleep at all, they do enter periods of rest. Some sharks need to keep moving to breathe, but others can remain stationary. Sharks that can stay still while resting include the whitetip reef shark, the Caribbean reef shark, the nurse shark, the wobbegong, and the lemon shark.

Some species of sharks have been observed to sleep in caves or on the ocean floor. In 1969, a free diver named Ramon Bravo discovered a group of streamlined sharks sleeping in a cave at Isla Mujeres, off the Yucatan Peninsula. This cave became known as the "Cave of the Sleeping Sharks" (La Cueva de los Tiburones Dormidos). The water inside these caves has an unusually high oxygen content and reduced salinity, which may be due to freshwater upwellings from the Mexican mainland. The sharks remained in the caves for hours, actively respiring by pumping water over their gills 20 to 28 times per minute.

Another example of sharks sleeping in caves is the whitetip reef shark, which is known to gather in large groups inside caves, piled on top of one another. These sharks are not particularly social, but they are not territorial and will willingly congregate in spots that offer shelter from larger predators. They often rest during the day and then hunt at night.

Lemon sharks have also been observed to rest on the ocean floor. They have been seen lying still on the open sand of the seafloor, propping themselves up on their pectoral fins and facing into the prevailing current to breathe more easily.

While it is unclear whether sharks truly sleep in the traditional sense, they do exhibit periods of reduced activity and motionlessness, which may be considered a form of sleep.

Frequently asked questions