Whales sleep with half their brain to avoid drowning

Few things are more relaxing than floating in a pool, hearing the sound of water, and resting your eyes for just a little while. For whales and dolphins, however, sleeping in the water is a bit more complicated. Indeed, whales do not have the luxury of floating on pool beds, and the sea is pretty dangerous when predators lurk in the dark. Unlike us humans, whales lack an unconscious breathing system. Cetaceans are voluntary breathers, which means that they need to think about every breath they take, 24/7. So how do these air-breathing mammals survive a night underwater? Short response: whales sleep with half their brain only!

Credit: Pixabay


Whales’ and dolphins’ sleeping strategy probably sounds familiar to anyone who struggled through boring school classes or other sleep fests. The trick is to keep one eye open to stay slightly alert while being half-asleep. Cetaceans have mastered this skill quite impressively. They have the incredible ability to shut down one of their brain halves and use the other to remember to breathe at the surface. Meanwhile, the eye connected to the awake hemisphere stays open to check for any obstacles, predators, or other problems. Whales and dolphins have taken the expression “to sleep with one eye open” quite literally.

How long do whales sleep?

Bottlenose dolphins and belugas can spend more than 30% of their day sleeping, while sperm whales only sleep for 7%. Grey whales are the sleepiest species, with up to 41% of resting behavior per day! Researchers concluded that different species might have various resting needs. Sleeping intensity may also vary among species; some may sleep quite profoundly, maybe even resting both hemispheres for a few minutes. Other species rest longer, but only with half their brain at a time.

Sleeping position

Most sleeping whales stay in a horizontal or vertical position close to the surface or at the surface. Resting at the surface is called “logging” because the motionless, floating body of a whale has an uncanny resemblance to a drifting log. Some species sleep at greater depths. For example, bottlenose dolphins in captivity sometimes sleep on the bottom of their tanks. 

Just keep swimming

A problem with living in water is that the body loses 90 times as much heat as it would in air. Keeping still for a long time can, therefore, dangerously cool down a whale’s body. A strategy that some species have evolved to avoid this is to keep swimming, even while they are sleeping. Many dolphins tag along with slow-swimming group members while one hemisphere rests. By “sleepwalking in the sea,” cetaceans can maintain body heat and stay with their group while sleeping.

Sleep like a baby

Just like human babies, newborn whales and dolphins need a lot of sleep and motherly care. During their first month, calves don’t leave their mother’s side. They eat, sleep, and swim in the mother’s slipstream. In fact, if the mother would stop moving, the calf would sink. Whales can keep afloat because of their fat, but the young calves’ baby-fat is not enough to keep them buoyant. For mom, this means non-stop swimming and alertness during the first weeks. Meanwhile, the newborn can enjoy the once-in-a-lifetime opportunity to have a long sleep and rest both brain-halves simultaneously. 


We don’t know for sure whether whales and dolphins dream. Interestingly, scientists found pilot whales and sperm whales that showed Rapid Eye Movements (REM). In humans, REM is a deep sleep state associated with dreaming. Does this mean that some whales can dream? And what would they dream of? Hopefully, future research will answer these questions. 

Sleeping underwater is not the most logical situation for an air-breathing animal. Yet, whales and dolphins manage to get their shut-eye with clever physiological and behavioral adaptations. So far, we know that many cetaceans either rest half of their brain or take short “cat naps” to breathe regularly. We can only dream of what other strategies less-studied species have developed to catch their z’s. 

Further reading

Eline van Aalderink is a recent MSc Marine Biology graduate from the University of Groningen (the Netherlands), where she specialised in marine mammal ecology and conservation biology. She is currently working as a marine mammal research assistant/supervisor at Archipelagos Institute of Marine Conservation in Greece.

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