How Do Whales Sleep? Vertical Naps and Half-Brain Rest

Why can’t whales sleep like we do?

People and other land mammals breathe automatically, even in deep sleep. Whales cannot. A whale’s blowhole stays shut by default, and the animal has to rise to the surface and open it each time it breathes, as described in How do whales breathe? A whale that slipped into deep, unconscious sleep far below the surface could, in theory, drown.

Sleep also has to fit around other needs. A resting whale must stay with its group, keep an eye on its calf, avoid predators such as orcas and large sharks, and hold on to body heat in cool water. Whales seem to solve these problems in different ways, and the answers vary between species.

Half-brain sleep in toothed whales

Most of what scientists know about whale sleep comes from toothed whales in captivity. When researchers recorded the brain waves of bottlenose dolphins, starting in the 1970s, they found that one half of the brain showed the slow waves of deep sleep while the other half stayed awake. After a while, the halves switched. This is called unihemispheric slow-wave sleep.

The same half-brain sleep has since been recorded in harbor porpoises, belugas, orcas and pilot whales. While one half sleeps, the eye on the opposite side usually closes and the other stays open, often watching nearby companions. The waking half keeps the animal surfacing to breathe and moving with its group. Toothed whales seem to have little or no REM sleep, the stage linked with vivid dreams in humans.

Dolphins are the best-studied example, covered in detail in How do dolphins sleep? In a 2005 study, newborn orca and dolphin calves in aquariums, and their mothers, showed almost none of the usual floating rest for weeks after birth, staying on the move around the clock. Later studies suggested that they still rest while swimming, but the question is not fully settled.

Sperm whales: naps in an upright position

The most striking whale sleep was described in 2008. Researchers studying tagged sperm whales found that the whales sometimes stopped swimming and drifted just below the surface, hanging vertically with their heads up or, less often, head down. These “drift dives” lasted minutes rather than hours and made up about 7% of the whales’ recorded time.

The researchers also met a group of whales in this state at sea. The whales hung motionless and did not react as the research vessel drifted among them, until the boat touched one of them. Such deep unresponsiveness suggests real sleep, possibly with both halves of the brain at once, unlike the half-brain sleep of dolphins.

No one has yet recorded the brain waves of a wild sperm whale, so this remains an interpretation. The upright posture may be the most stable and effortless position for a heavy whale with a large, oil-filled head, and it keeps the blowhole close to the surface for the next breath.

How do baleen whales rest?

Far less is known about baleen whales such as humpback, blue and right whales, which are too large to keep in captivity. Observers often see them “logging”, floating motionless at the surface like a log, with the back and blowholes just above the water. Right whales and humpbacks are commonly seen resting this way.

On the breeding grounds, whale watchers often see humpback whale mothers hanging almost still just below the surface while their calves rise regularly to breathe. During migration, humpbacks and gray whales travel thousands of kilometers, often slowly and steadily, and researchers think they may rest while swimming. Whether baleen whales use half-brain sleep, deep sleep or both is still unknown.

How much do whales sleep?

There is no single answer. Captive bottlenose dolphins spend roughly a third of the day in half-brain sleep, spread over many short bouts. Tagged sperm whales spent only about 7% of their time in drift dives, although they may also rest in other ways that tags cannot detect.

Seals offer a useful comparison. In 2023, scientists recorded the brain waves of wild northern elephant seals diving off California. The seals took naps of less than 20 minutes during deep dives, often while spiraling slowly downward, and averaged only about 2 hours of sleep a day during months at sea. Whales living in the open ocean may face similar pressures to sleep little and in short bursts.

How do scientists study whale sleep?

In aquariums, researchers can record brain waves with electrodes, watch which eye is open and film resting postures day and night. This work, mostly on dolphins, belugas and orcas, revealed half-brain sleep.

Wild whales are much harder to study. Suction-cup tags record depth, movement and sound for hours at a time, and they show when a whale stops swimming and clicking, as in the sperm whale drift dives. Drones and long-term photo studies add more clues. Brain recordings like those made on elephant seals have not yet been possible on wild whales, so how the largest animals on Earth really sleep is still partly a mystery.