Sleep is something that most complex animals on Earth tend to do. Humans spend about a third of their lives in this unconscious state. Though our busy lives may at times prevent us from getting optimal sleep; no matter how hard we try to resist, sleep is inevitable. This is because sleep is vital for human health. Research has elucidated that sleep carries out important tasks which support brain health and discoveries continue to surface. Little is known, however, about the sleep neurophysiology of our mammal cousins, whales.
In captivity, it was observed that killer whales slept up to eight hours a day, but it was not until 2008 that we had any conclusive evidence of a whale species sleeping in the wild. How do they not drown? How do they avoid predators? Are they really sleeping or it something else? Do they dream? In the sections that follow, we will explore whale sleep and explore those very questions.
Cetaceans and the Mammal Classification.
Despite distinct differences, whales, dolphins, narwhals, and porpoises are all related under the order Cetacea. Though often confused as fish, they are exothermic mammals, like cows, humans, and cats. Cetaceans have flippers, a streamlined shape, are fully aquatic, and their body is wrapped in a layer of dense fat called blubber.
Evolution Reflection
It is commonly hypothesized by evolutionary biologists that early cetacean ancestors began as terrestrial mammals that eventually returned to the sea, probably due to food abundance. In adapting to their new aquatic environment, early cetaceans lost their body hair, developed blubber, and became more streamlined. Outward testicles and mammary glands became surrounded and protected by tissue. We see the same happening with hippos today. Though all animals are technically in a transitory state, hippos, are a good example of some of the required transitions needed to head back into the sea. Hippos, in fact, sleep underwater but unconsciously bop up to take a breath and then release their air and re-submerge. This has enabled hippos to well... not die. How did their closest cousins, the cetaceans, evolve their sleeping behavior? Unihemispheric sleep.
What is Unihemispheric Sleep?
If whales or dolphins try to fall asleep the way humans and most other animals do, they would drown. Due to the need for air, whales cannot become unconscious for long periods of time, but they still require sleep. This sounds like quite a bit of a conundrum. Thankfully, whales sleep unihemispherically.
Unihemispheric sleep is when a brain goes into a sleep state but only one side at a time. Not only does this keep the whale from drowning, but it also keeps them aware of their surroundings, and able to move about while in the water. This form of sleeping also allows them to avoid predators, which otherwise would be near impossible to do while sleeping. Humans don't exhibit this form of sleep usually but a similar form of unihemispherism can occur when a person is in a new environment and having trouble sleeping (spontaneous dynamic symmetry).
Breathing and Sleep
Whales tend to sleep vertically (although they have been spotted sleeping horizontally as well). When observing whales for an extended period of time, you will periodically see them bobbing in the water. Sound familiar? Exactly! This happens because although they are sleeping, they will still rise to the surface every so often to breathe before floating back down. The whales will stay submerged for about 35 minutes before repeating, at least in larger whales. Baby whales may have to surface every few minutes due to needing air more frequently.
When sleeping, larger whales can experience a form of sleep apnea (a pause in breathing) and a reduced heart rate. After holding their breath for extended times, they will surface and begin breathing rapidly to recover oxygen reserves. During sleep, their heart rates will reduce from 60-90 BPM down to 20-40 BPM. While sleeping, blood only pumps to areas of the body that needs it most, like the heart and brain, while other things, like digestion, are made to wait. This also helps to preserve oxygen.
In an encounter with sperm whales in 2008, researchers discovered that, sometimes, whales may go into full sleep. This was realized when researchers, after cutting their boat's engine, accidentally bumped into a sleeping sperm whale head-on. This suggests that they may sometimes go into short bursts of full sleep instead of always being at half-sleep. More evidence is needed, however, to support this hypothesis.
Despite running into them while sleeping, the researchers discussed that sperm whales may be one of the least sleep-dependent animals known, only spending 7% of their time sleeping. This is different than most other whales and dolphins, who spend closer to a third of their hours asleep.
Mother and Child
How did this unihemispheric function evolve? Baby whales and dolphins (known as calves) are unable to stay afloat for the first few weeks of their lives. Mothers have to pull their calves along in what is known as a slipstream. The calf benefits hydrodynamically from being so close to the mother and is thus able to conserve energy. This is known as echelon swimming. Depending on the species, calves remain very close to their mothers for weeks. At least until the calf has fed on enough milk to develop its buoyancy-sustaining blubber. Until then, the mother cannot stop moving or else the calf risks slowly sinking to the bottom. Evolving a method to breath and sleep while on the move was imperative to the evolution of early cetaceans.
Dream a Little Dream of Krill?
Can whales dream? It seems feasible. They are mammals after all and most of us are familiar with Youtube videos showing cute pups running in their sleep. Not so fast, though. Evidence suggests that REM sleep is strongly correlated with the dream experience. It is difficult to research sleep in cetaceans, especially whales, so there is little evidence that they dream. That said, there was an observation of a single pilot whale that showed the tell-tale twitching markers of REM sleep which lasted for 5 minutes.
Dolphins make similar twitches and are also known for talking in their sleep. A study of 5 dolphins at the Planète Sauvage dolphinarium in France has shown that dolphins mimic various sounds in their sleep. This includes humpback whale noises that they heard over the program speakers during the day.
Even though there is some evidence that cetaceans may dream, the Encyclopedia of Sleep, 2013, keeps us grounded with the statement below:
"Despite long periods of electrophysiological studies of sleep, no rapid eye movement (REM) sleep episodes, typical of terrestrial animals, have been observed in any of the studied cetacean species. Muscle jerks, body twitches, and REMs (all features of REM sleep) have been documented in behaviorally resting cetaceans... Based on these characteristics, muscle jerks in cetaceans fit the behavioral features of REM sleep in terrestrial mammals. However, a significant portion of jerks occur at the beginning of the sleep periods and during quiet waking. Based on the available data, REM sleep is either absent in cetaceans or is present but has taken on a modified form that has escaped detection."
Captivity vs the Wild
With the above statement in mind, it is important to consider that the sleep patterns of animals in captivity may differ from sleep patterns in the wild. Most of what we know about cetacean sleep is from members in captivity. Future studies may reveal to us that, just as whales use their brains differently when sleeping, so too may be the case in how they dream.
Learn more about whale sleep and watch whales in action by checking out the video below!