The short answer is that they are conscious breathers and therefore sleep in different ways than land mammals (like us). We are unconscious breathers, so our bodies automatically breathe to take in air even when we are sleeping. Cetaceans are conscious breathers, meaning that they have to make a decision on when to breathe. This might seem complicated for an animal that spends all of its time in the water, but whales and dolphins are experts and are well-adapted to spending their entire lives in the ocean.
All whales and dolphins sleep, but different species have different methods and requirements for sleep and rest. The length of sleep can vary massively between species. There are some common methods and positions for sleeping. These include simply resting quietly in the water, either horizontally or vertically, or sleeping while slowly swimming next to another member of their pod or in small groups. Dolphins in captivity have been recorded sleeping for brief increments of time at the bottom of their tanks. Humpback whales are often found resting motionless on the surface of the ocean while sleeping. They cannot sleep for much longer than 30 minutes without risking lowering their body temperature due to inactivity.
A very common assumption is that whales sleep with half of their brain ‘shut off’ and one eye closed. The theory is that they do this to maintain an awareness of potential predators or threats that may approach. It is thought that this also allows them to remember to breath at the right time. This behavior has been reported in many different types of dolphins, who can sleep for 2-4 hours at a time. Some dolphins sleep for roughly 33% of the day, while the larger sperm whale is thought to sleep for only 7% of the day!
Boating encounters with sperm whale pods suggest that they enter a deeper sleep than dolphins. In 2008, a small group of scientists working off the coast of Chile happened to encounter a pod of sleeping sperm whales. They were working to record sperm whale calls and were below deck with the engine off when they discovered that they had drifted right into a pod of sleeping sperm whales. It was not until the boat accidentally nudged one of the sperm whales that they noticed the presence of the boat. This is suggestive of a deeper sleep with less acute awareness. The sperm whales swam off and resumed their sleeping.
It is notoriously difficult to study cetacean sleeping behavior in the wild. There is still much to learn about the sleep requirements and patterns of whales and dolphins. We have encountered sleeping whales before on our Whale Watching tours, though it is not common. One one of our morning tours in April, 2015, our boat came across a sleeping humpback. We observed the whale resting just below the surface, coming up every few minutes to breathe. It did not seem to notice the boat and we had the chance to closely watch this gentle giant – a unique and exciting experience!