Sperm whales are the largest toothed whales and are known for their extreme diving abilities. They often go down to 3,000 feet in search of squid, their primary food source. These dives can last up to an hour, requiring the whales to hold their breath for extended periods, resurfacing briefly only to exhale, inhale, and dive again. This lifestyle, however, leaves little time for the kind of extended sleep seen in land animals.

While dolphins and belugas can spend more than 30% of their day sleeping (like humans) sperm whales only sleep for 7% of the time. This sleep happens in brief naps that usually last around 10 minutes. It gets even more interesting: they sometimes sleep vertically, like this:

Sperm whales sleep vertically

In the wild, several types of whales have been spotted sleeping horizontally, close to the surface. But sperm whales seem to enter a deeper state of sleep. This would also explain how they manage with so little sleep — but the curious part is they also do it vertically.

Patrick Miller is one of the researchers who published a groundbreaking study on sperm whales. With colleagues, he recorded the behavior of 59 sperm whales around the world, using devices connected with suction cups. The scientists found that sperm whales seem to engage in a deep, “drift dive” type of nap, where they drift to a vertical position and remain immobile.

“One exciting aspect of this finding is that it suggests that the largest toothed whale actually might sleep in a fashion that we recognise as similar to sleep in terrestrial mammals.  The sperm whale behaviour we describe seems to allow normal-looking quiescent sleep, possibly including REM sleep which has never been clearly observed in any cetacean,” he said.

More weird sleeping habits

Unlike humans and other mammals that need extended, consolidated sleep, marine mammals face different challenges. They live in an environment where they need to be alert, breathe at the surface, and cover vast areas in search of food. As a result, they’ve developed unique sleep habits.

Dolphins, for example, alternate sleep between brain hemispheres, a phenomenon called unihemispheric slow-wave sleep, allowing them to rest while remaining aware of their surroundings. Until now, researchers assumed sperm whales had a similar sleep pattern.

Dolphins, for example, are known to sleep by shutting down one hemisphere of their brain at a time, a process called unihemispheric slow-wave sleep. This allows them to rest while remaining partially aware of their surroundings. However, sperm whales—a species distinct in size, diet, and habitat—have developed this unique approach.

Traditionally, people assumed that sperm whales also have this sleep pattern where they shut off half their brain at a time. But Miller’s research suggests something different. During this vertical sleep period, sperm whales remain motionless, entering a state of reduced consciousness.

“Our study presents the first evidence that whales in the wild may sleep with both sides of the brain (bihemispheric). This is supported by the fact that the whales we approached did not react to the boat even though it was visible to both eyes. From what we have found, the amount of time sperm whales seem to have for sleep is very little, so it would make more sense for them to sleep both sides at a time.”

The study was also important because it suggests that cetaceans in the wild may sleep very differently from those in captivity. This means that studies conducted on captive creatures have to be taken with a big grain of salt.

There’s much we don’t know yet

One of the primary reasons scientists believe sperm whales sleep vertically is to minimize their vulnerability to threats. Sleeping vertically may be a compromise between rest and readiness, allowing the whales to stay in a position that offers a quick escape if danger arises. While sleeping at the surface could expose them to predators or passing ships, drifting vertically within the water column may provide a safer alternative.

Vertical sleep could also be a way for these massive animals to conserve energy. By remaining mostly motionless and drifting in a vertical orientation, sperm whales likely reduce the muscular strain of actively maintaining a horizontal position. This is especially beneficial given the energy demands of their deep dives and constant need for food, particularly in the nutrient-scarce regions of the open ocean where they hunt.

But ultimately, there’s a great deal we don’t know about how sperm whales (and other cetaceans) sleep — because it’s so damn difficult to study them. For instance, scientists still debate whether sperm whales experience true REM sleep, a deep sleep phase associated with dreaming and memory processing in humans and some other animals. It is also unclear if sperm whales experience unihemispheric sleep like dolphins or if they enter a full, albeit brief, state of unconsciousness.

Advances in technology have transformed our ability to study sperm whales, enabling scientists to uncover previously hidden aspects of their lives. With tools like underwater drones, high-definition cameras, satellite tagging, and other sensors, we can now observe these deep-diving giants in their natural habitats. As technology progresses, we’re inching closer to answers, but the complex world of these enigmatic creatures still holds countless secrets waiting to be uncovered.