Research published July 23, 2026 in the Journal of Experimental Biology has identified the mechanism that allows sperm whales to sleep in a vertical position just beneath the ocean surface. A team from the University of St Andrews and Université de Neuchâtel found that the whales release gas bubbles during rest to regulate their buoyancy, counteracting the positive buoyancy that would otherwise pull them upward (University of St Andrews).

Sperm whales are the only whale species known to rest vertically. They nap for 10 to 15 minutes at a time underwater, heads pointing upward, in a posture thought to buffer them from surface wave action (Popular Science). The vertical orientation places them just below the surface, but maintaining that depth is not passive. Sperm whales carry large volumes of spermaceti oil in their heads, making them naturally positively buoyant. As breath-hold divers resting in shallow water, they face expanding gas in their lungs that would cause them to drift upward unless actively managed.

The whales manage this by releasing bubbles. Professor Patrick Miller of the Sea Mammal Research Unit at the University of St Andrews described the process: sperm whales "exquisitely control their buoyancy by releasing gases to remain submerged with near-neutral buoyancy just below the sea surface while asleep." The bubble release reduces the whales' overall gas volume, trimming their positive buoyancy enough to hold station at depth without swimming.

The findings rest on two data streams. Researchers attached small suction-cup tags to sperm whales off the Norwegian coast; the tags recorded sound and three-dimensional movement. Clear bubble sounds were captured in the acoustic data. The movement data was then used to build a simulation incorporating tissue density, hydrodynamic drag, and gas volumes within the whales' bodies. The simulation confirmed that bubble release reduces positive buoyancy sufficiently to keep the animals submerged while resting.

One detail from the tagging data adds a layer of nuance: sperm whales begin resting dives with less diving gas volume than they carry for deep foraging dives. This means they arrive at their resting depth already carrying less gas, and the bubble release serves as a fine-tuning mechanism rather than a large-scale correction. The combination of lower initial gas volume and controlled bubble release brings them to near-neutral buoyancy at the surface.

The researchers also suspect that the bubble release could be connected to off-gassing of excess CO₂ or N₂ from tissues into the lungs, which would carry implications for metabolic gas exchange during sleep. This aspect is not yet confirmed and is flagged by the authors as a hypothesis.

The instrumentation behind the study is worth noting for those who follow marine biologging. Suction-cup tags that record synchronized multi-axis accelerometry and acoustics have become a standard tool in cetacean behavior research, but deploying them on free-ranging sperm whales in North Atlantic conditions remains technically demanding. The simulation approach, combining empirical tag data with a physical model of body density and drag, is a useful template for testing buoyancy hypotheses that cannot be addressed through observation alone.

The broader context here is how little we still understand about sleep in large marine mammals. Sperm whales are among the largest predators on Earth, and the mechanics of how they rest, for such brief intervals, in an environment where buoyancy and breathing are in constant tension, has been an open question. The vertical sleep posture itself was only documented relatively recently in the wild. This study closes one part of that gap, the buoyancy-control mechanism, while opening another: whether the bubbles serve a metabolic function beyond ballast.

In this author's view, the elegance of the mechanism is its simplicity. No special anatomy is invoked beyond what sperm whales already possess. The whales use a byproduct of respiration, gas, as a ballast adjustment tool, turning a potential problem, positive buoyancy from expanding lung gas, into a self-correcting system. It is a solution shaped entirely by natural selection, operating without conscious effort during an unconscious state. That is a narrow but genuinely interesting result, and the gas-exchange hypothesis gives the next team of researchers a clear line of inquiry.