Scientists have figured out how sperm whales manage to sleep underwater without floating to the surface. The answer, published July 23, 2026 in the Journal of Experimental Biology, is that the whales release gas bubbles while they rest, using them to control how high or low they sit in the water (University of St Andrews).

Sperm whales are the only whale species known to sleep standing upright in the water. They nap for 10 to 15 minutes at a time, heads pointing upward, just below the ocean surface. Researchers think this upright position helps protect them from waves (Popular Science).

But staying at the right depth is not as simple as it sounds. Sperm whales carry large amounts of a waxy oil in their heads called spermaceti, which makes them naturally float upward. On top of that, the air in their lungs expands when they are in shallow water, pushing them even more toward the surface. Without a way to manage this, they would drift up while asleep.

The whales solve the problem by releasing bubbles. Professor Patrick Miller of the University of St Andrews explained that sperm whales "exquisitely control their buoyancy by releasing gases to remain submerged with near-neutral buoyancy just below the sea surface while asleep." In other words, by letting out small amounts of gas, they reduce the upward push just enough to stay put without swimming.

The research team gathered evidence in two ways. First, they attached small suction-cup tags to sperm whales off the coast of Norway. The tags recorded sound and movement in three dimensions. The sound recordings picked up clear bubble noises. Second, the researchers used the movement data to build a computer model of the whale's body, including its density, how it moves through water, and how much gas it carries. The model confirmed that releasing bubbles reduces the upward pull enough to keep the whales submerged while they sleep.

One extra detail adds a layer of interest. The tagging data showed that sperm whales start their resting dives carrying less air than they do when they dive deep to hunt for food. So they arrive at their sleeping depth already holding less gas, and the bubbles serve as a small fine-tuning adjustment rather than a big correction.

The researchers also think the bubble release might help the whales get rid of extra gases like carbon dioxide or nitrogen that build up in their bodies. This idea is not confirmed yet and the authors describe it as a hypothesis, meaning a proposed explanation that still needs testing.

The broader context here is how much we still do not know about sleep in large ocean mammals. Sperm whales are among the biggest predators on Earth, and how they manage to rest in short bursts, in an environment where floating and breathing are always working against each other, has been a puzzle. The upright sleep position itself was only first seen in the wild relatively recently. This study answers one part of the question — how the whales control their depth while sleeping — but opens another: whether the bubbles also help with something beyond staying level in the water.

In this author's view, what makes this finding appealing is its simplicity. The whales do not need any special body part to do this. They use gas, which is just a byproduct of breathing, as a tool to adjust their weight in the water. They turn a problem, the tendency to float upward, into a self-correcting system that works even while they are unconscious. It is a small but genuinely interesting result, and the idea that the bubbles might also help with gas exchange gives future researchers a clear next step.