Marine Mammals: No Rem Sleep, Why?

Marine mammals, such as dolphins, have a unique way of sleeping called unihemispheric slow-wave sleep (USWS). This is when half of the brain goes into a deep sleep while the other half remains awake, allowing them to rest while still being able to breathe, move, and keep an eye out for predators. This is in contrast to the sleep of terrestrial mammals, which typically involves both hemispheres and is characterised by two distinct stages: slow-wave sleep (SWS) and rapid eye movement (REM) sleep. While the function of sleep is not fully understood, it is essential for the health of animals.

Unihemispheric sleep

• Amazon river dolphin
• Beluga whale
• Narwhal
• Bottlenose dolphin
• Pacific white-sided dolphin
• Pilot whale
• False killer whale
• Porpoise
• Sperm whale
• Northern fur seal
• Southern sea lion
• Steller sea lion
• Harbour seal
• Caspian seal
• Harp seal
• Northern elephant seal
• Baikal seal
• Walrus

It has also been observed in the following birds:

• Common blackbird
• Domestic chicken
• Glaucous-winged gull
• Japanese quail
• Mallard
• Northern bobwhite
• Orange-fronted parakeet
• Peregrine falcon
• White-crowned sparrow

REM sleep is absent in cetaceans

REM sleep has been recorded in all land mammals. It is characterised by cortical arousal, rapid eye movements, muscle jerks, muscle tone suppression, and increased heart and breathing rate variability. However, the only report of REM sleep in cetaceans was a 6-minute episode in a pilot whale.

The need to come to the surface to breathe may make the physiological process of breathing more complicated and incompatible with deep bilateral slow-wave sleep. Alternating REM with NREM sleep conserves energy by decreasing the need for core body temperature defence.

The risk of predation could be an important factor in the very early days of cetacean aquatic evolution when their ancestors started living in the shallow warm waters of the Tethys sea. At this stage, they were not large pelagic animals and possibly lacked the abilities to hold their breath for long periods and to dive to depths sufficient enough to avoid potential predators. Therefore, consistent motion, monitoring of the environment for predators and conspecifics to maintain group coherence may have been pressures with enough significance to lead to the form of sleep seen in modern cetaceans.

The third alternative is the need to maintain heat production in the face of the thermal challenge of living in water. Being mammals, cetaceans are under a substantial pressure to avoid heat loss, and the altered form of sleep seen in extant cetaceans is consistent with the ability to increase heat production. It is also consistent with known changes in the environment of the cetaceans over time.

Cetaceans' sleep is compatible with echolocation

Cetacean sleep is compatible with echolocation because it allows cetaceans to maintain movement and continuous vigilance, while allowing slow waves to be present in at least half of their brain at a time.

Cetaceans are fully aquatic marine mammals that have evolved about 50 million years ago. They are known to have unihemispheric sleep, which is a core feature of cetaceans sleep. Unihemispheric sleep allows cetaceans to surface without awakening and for Otariidae seals to maintain a characteristic posture at the surface. Movement during unihemispheric sleep also facilitates muscle thermogenesis and postural thermoregulation. This pattern can be achieved only in open (ice-free) waters, which is the typical habitat for the majority of cetaceans and all otariids.

The open eye of cetaceans during sleep can be directed to their schoolmates or to the remaining pod, allowing them to maintain visual contact with conspecifics or detect predators. This is known as the sentinel function.

REM sleep is suppressed in fur seals in water

The northern fur seal (Callorhinus ursinus) is a semi-aquatic mammal that can sleep on land and in seawater. When sleeping in water, fur seals show non-REM sleep in only one brain hemisphere at a time, just as dolphins do. They adopt a rather odd sleeping posture, with the flipper attached to the waking part of the brain staying in motion in the water to maintain its position, while the other flipper is taken out of the water and 'rests'. The eye attached to the waking part of the brain is aimed into the water, where sharks and other predators come from, while the other eye is closed.

Fur seals can go without REM sleep for days or weeks when in seawater, where they spend most of their lives. After this nearly complete elimination of REM sleep, they display minimal or no REM sleep rebound upon returning to land. This challenges the idea that REM sleep is homeostatically regulated. Instead, it supports the hypothesis that REM sleep serves to reverse the reduced brain temperature and metabolism effects of bilateral non-REM sleep, a state that is greatly reduced when the fur seal is in seawater.

The ability of fur seals to "naturally" eliminate or substantially reduce the amount of REM sleep for days or weeks in water leads to the question of whether REM sleep is vitally needed. It appears that giving up REM sleep in water is more beneficial for the fur seal than having REM sleep to fulfill some yet unknown vital function. This is unless REM sleep serves different functions in different animals.

REM sleep in fur seals is eliminated at a time when the animal requires high levels of alertness, performance, learning, and motor activity to navigate, locate prey, and avoid predators compared to when the seal is resting on land. This contradicts the idea that REM sleep is essential for maintaining cognitive functions.

There is some evidence that fur seals can have a reduced amount of REM sleep for up to 1-2 months. However, it is unknown what happens to REM sleep when they are aquatic for 6-10 months.

REM sleep is suppressed in marine mammals

Marine mammals, such as dolphins, have a unique sleep pattern called unihemispheric slow-wave sleep (USWS). During USWS, one half of the brain remains awake, allowing the animal to breathe, move, and remain vigilant of potential dangers, while the other half enters slow-wave sleep. This is in contrast to the sleep patterns of terrestrial mammals, which exhibit two distinct sleep stages: slow-wave sleep (SWS) and rapid eye movement (REM) sleep.

REM sleep, which is characterized by increased brain activity, eye movements, and muscle paralysis, is absent or negligible in dolphins and other cetaceans. This may be due to the incompatibility of REM sleep with the aquatic environment, as it would leave the animal immobile and vulnerable to predators. Instead, dolphins primarily experience USWS, which enables them to maintain essential functions like breathing and movement while still getting the necessary rest.

The suppression of REM sleep in marine mammals is likely an adaptation to their aquatic environment. By prioritizing USWS, dolphins can ensure their survival by remaining vigilant and responsive to their surroundings, even while sleeping. This unique sleep pattern allows dolphins to rest while also attending to their immediate needs, such as breathing, thermoregulation, and predator avoidance.

While the exact mechanisms behind the suppression of REM sleep in marine mammals are not fully understood, it is clear that they have evolved unique sleep patterns to suit their aquatic lifestyle.

Frequently asked questions