How Do Whales Sleep
How do whales sleep without drowning? Surely this presents a major difficulty for air-breathing mammals who spend their entire lives in the ocean.
It's true that sleeping presents an unusual challenge for dolphin, porpoise, and whale species. Their common ancestry with land mammals makes them unable to extract oxygen directly from the water.
Whales and dolphins have evolved a range of strategies for sleeping safely. Read on for an in-depth look into how cetaceans simultaneously catch some shut-eye, breathe, and avoid predators!
How Do Whales Breathe When They Sleep?
A common assumption, and not an unreasonable one, is that whales simply sleep at the water's surface so they can continue to breathe while getting their rest.
However, the very nature of breathing in whales and dolphins prevents such a straightforward solution.
Humans and other land mammals are known as "unconscious breathers" because our breath continues automatically without us thinking about it.
Our breathing rate is controlled by physiological feedback systems. That said, we can impose conscious control over our breathing when we choose to.
One example of this is holding your breath when underwater. Whales and dolphins are conscious breathers which means they are constantly aware of their breathing, need to actively decide when to take each breath, and must come up to the surface of the water for each inhalation.
Sleeping as we would consider it is therefore not really an option for cetaceans. So just how do whales sleep without drowning?
Unihemispheric Sleep
A study originally carried out on bottlenose dolphins discovered a neat trick that helps explain how these animals rest while remaining alert enough to breathe when needed, avoid predators, and generally preserve their wellbeing.
In this species of dolphins, sleep was shown to be achieved in one part of the brain at a time. Like humans and many other animals, dolphin brains are split into two sides (hemispheres). By "switching off" one hemisphere of their brain at a time, dolphins are able to rest one half of the brain while the other side takes care of their survival needs. After a couple of hours, the two sides switch over and it is the turn of the other half to sleep.
Unihemispheric sleep has now been studied in other species of whales and dolphins such as orcas. If it wasn't for this adaptation, the closest dolphins and small whales would get to sleeping would be cat-napping near the surface in between breaths.
Breath Hold Sleeping
Some larger species such as humpback whales and sperm whales can hold their breath for long enough to obtain deeper sleep than their smaller cousins. Sufficient lung capacity to go for 90 minutes or more between breaths lets these large whales sleep underwater resting their whole brain before they need to surface for air. This is more efficient than only half the brain being able to snooze at any one time.
Mothers of Infant Calves
If you're a parent you'll recall the trials of those first few weeks/months of disrupted sleep. If you think that was bad, spare a thought for whale mothers.
In many species, whale calves require constant support from their mother in the first month of their life. They are not yet strong enough to swim independently and have insufficient blubber to maintain buoyancy.
Because of this, the mother swims constantly, creating a slipstream in which the calf can swim and nurse while building up its strength and reserves. This behavior is known as "echelon swimming" and has been observed in several species, including orcas and humpback whales.
How Long Do Whales Sleep?
Since there are two types of sleep found in different species of whales, how long whales sleep underwater tends to depend on whether they are unihemispheric sleepers or rely on full sleep.
Sperm whales sleep less than most other species. Despite being able to hold their breath for well over an hour, sperm whales are generally only recorded sleeping for around 15 minutes at any one time.
They will do this at some point in most breath cycles but the short duration means they still only spend around 7% of their day asleep.
At the other end of the scale, Beluga whales (an animal more closely related to orcas and dolphins) can practice unihemispheric sleep for hours at a time, and up to 8 hours a day in total. This means they spend around 30% of their time in a partially conscious resting state.
Where Do Whales Sleep?
The short answer is whales generally sleep near the surface of the water, so they can rise to breathe when necessary.
Sleeping on the Go
Dolphins and the other species who engage in unihemispheric sleeping tend to continue swimming slowly while asleep. They will often do this in pairs or a small group, with younger offspring in the center to make them less of a target for potential predators.
In general, it is best for whale species to remain moving as swimming helps stop them from losing too much body temperature to the water and also makes breathing easier for many whales.
Although the smaller whales prefer not to stop swimming, at night time some of these species can be found resting motionless on the surface in a practice called "logging". This is thought to facilitate a deeper sleep than resting on the move, although it could result in a dangerous drop in body temperature if maintained for too long.
Vertical Sleeping
Some of the largest animal species in the whale family, including sperm whales, have been observed resting in a vertical position in the water. The reason these sleeping giants adopt this strange orientation is not entirely clear, although some scientists have posited these "sleeping" sperm whales may be resting while partially alert and able to keep an eye out, very similar to the unihemispheric sleeping habits of dolphins, orcas, and belugas.
Predators lurk in the depths, and even an animal as large as a sperm whale has to be cautious when exposed near the surface.
Final Caveat
We know for sure that whales sleep in some form, although theories are often extrapolated from anecdotal evidence. For example, a research team studying sperm whale calls drifted into a pod but the sleeping sperm whales didn't notice until the boat accidentally nudged one of them.
The current ideas around questions such as "How do whales sleep without drowning?" and "How long do whales sleep?" are based on a small number of studies. It's difficult to conduct robust research on sleeping sperm whales, for example, and almost impossible to get the ideal sorts of data such as EEG/ECG traces due to the size and oceanic nature of these magnificent animals.
Hopefully, with improvements in research methods and technology, we will soon know more about these sleeping giants!
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