A growing body of research is revealing surprisingly human-like characteristics in sperm whales (Physeter macrocephalus)—may have an alphabet and even use vowels like humans in their communication. Now, researchers have investigated another very familiar behavior in this marine mammal: sleeping.

Researchers theorize that sperm whales nap for 10 to 15 minutes at a time underwater, in an upright position with their heads pointing upward. Sleeping this year, they are out of reach of the tumultuous surface waters, but close enough to emerge when they need to take a breather. But their heads are huge and floating, so how do these cetaceans avoid floating while they sleep? The researchers write in a Journal of experimental biology The article suggests the answer is to blow bubbles.

The main author, Noémie Freymond, tells popular science That co-author, Patrick Miller, “provided the first detailed description of sperm whale resting behavior. He observed that sperm whales release bubbles while sleeping, but it had never been shown why they do so.” Freymond conducted his master’s thesis research on the resting behavior of the sperm whale. He is currently a doctoral student at the University of Neuchâtel, where he studies chimpanzee sleep. Miller is a researcher in the School of Biology at the University of St Andrews.

The team monitored 42 sperm whales in Norway’s Lofoten Islands using devices that record the whales’ bubble sounds. They noted each whale’s depth and body orientation, which indicated when the whales were sleeping. The devices were sucked up by the whales and the team collected them after they detached from the whales and floated to the surface of the water. Then he went to the laboratory.

Sperm whales can rest in three different ways. They can sink slowly to about 26 feet deep, tail first. They can also dive, headfirst, not far before their floating heads reappear. Or they can dive deep (over 656 feet) and rest as they float back up.

In this context, the team found that whales that rested closer to the water surface released more bubbles while they slept. Specifically, they blew bubbles about 11 times during each nap. Sperm whales resting while floating from greater depths blew bubbles only three or four times during their ascending nap.

Blowing bubbles allows sperm whales to control their buoyancy, researchers say. Whales that sleep near the surface of the water expel some of the air from their lungs, preventing them from floating again. Deep dive naps require more oxygen, so you would think they would have to release more air to prevent buoyancy, but that’s actually not how it works. Water pressure at greater depths makes whales less buoyant, even if they have more air in their lungs, so they don’t need to release as many bubbles to stay underwater.

“We were lucky to have access to such a large data set with many tag deployments, because the main challenge is that a deployment has to last long enough to capture the behavior at rest,” explains Freymond. “This allowed us to show that sperm whales release bubbles to regulate their buoyancy while resting.”

What the team can’t yet say for sure is whether sperm whales blow bubbles while they sleep consciously or unconsciously. When a dolphin sleeps, half of its brain is still awake. Is this what sperm whales do too? Or are they completely asleep, like humans? After all, releasing air to control buoyancy requires an awareness of the environment.

It remains to be seen what future research will burst this bubble. More importantly, scientists should not investigate whether whales blowing bubbles in their sleep is considered cetacean snoring.

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