Cetaceans, or whales and dolphins, have a pretty unique way of getting some shuteye. These marine mammals actually only rest one half of their brain at a time when sleeping, in what is known as unihemispheric slow-wave sleep. While one hemisphere of the brain rests, the other remains alert. The hemispheres are alternated between sleep and consciousness a number of times in each resting period, which can last anywhere between a few minutes and a few hours.
There are a number of reasons why such a form of sleep makes sense for cetaceans. Unlike humans and other animals, cetacean breathing is voluntary. This means that they need to consciously think about every breath they take, and that they cannot breathe when unconscious. This was discovered when the first captive dolphins were given anaesthetics and subsequently died after they stopped breathing. By resting only one half of their brain at a time, cetaceans can remain conscious at all times in order to breathe. Remaining conscious is also essential in order to allow cetaceans to keep swimming while sleeping so that they are able to rise up to the surface in order to breathe. Furthermore, this unihemispheric sleep allows them to remain alert to their surroundings and any potential predators, as one eye remains active while the eye associated with the sleeping brain hemisphere is shut.
Cetaceans generally sleep in one of two ways. Some cetaceans rest by remaining almost motionless in the water, with their bodies hanging either horizontally or vertically in the water column or floating at the water’s surface. An example of this form of sleep behaviour can be seen in sperm whales, where a number of whales will hang vertically in the water column as they rest. Other cetaceans rest while swimming slowly alongside one another, often with specific breathing patterns. This can be seen in killer whales, where individuals in a group form “resting lines” that surface to breathe five or six times during short dives before taking a long dive of between three to five minutes.
The evolution of unihemispheric sleep in cetaceans is just one of the many amazing ways that these aquatic mammals have adapted to life at sea.