Sperm whales can rest in one of the ocean’s strangest positions: nearly vertical, suspended just below the surface and sometimes gathered in small groups. For years, the image was more famous than the mechanism. A July 2026 study now offers an answer to the basic physical puzzle: the whales release bubbles to reduce their positive buoyancy and keep from rising too quickly while they rest.
The finding does not prove every detail of sperm whale sleep. Researchers still do not know whether the animals fully shut down both brain hemispheres, and the familiar 7.1% figure measures tagged time spent in drift-dives rather than a complete daily sleep budget. What the new work does show is that the apparent stillness is being precisely managed.
The bubbles hidden inside a silent rest
In a 2026 Journal of Experimental Biology paper, Noémie Freymond, Alec Burslem and Patrick Miller analysed data from suction-cup tags placed on 42 sperm whales off northern Norway. The tags recorded sound, depth and three-dimensional movement, allowing the team to identify resting dives and hear bubbles leaving the blowhole. The researchers then matched a physical simulation to depth profiles from 10 tagged animals that rested mainly by diving head-down.
All of the modelled head-down resting whales released bubbles. The simulation indicated that those releases reduced positive buoyancy and helped the animals remain submerged, while the whales also appeared to begin resting dives with less gas than they carried into deep foraging dives. Popular Science’s report on the study described the same dataset and noted that the animals used several resting patterns rather than one rigid pose.
The posture was documented long before the mechanism
The landmark 2008 Current Biology study by Miller, K. Aoki, Luke Rendell and M. Amano used suction-cup tags to describe passive, shallow drift-dives in stereotyped vertical postures around the world. In one filmed encounter off northern Chile, a group of vertical whales did not respond normally to a closely passing vessel until contact occurred. That unusual lack of response was one reason the researchers interpreted the behaviour as sleep.
The study reported drift-dive bouts during 7.1% of recording time, or 36.7% of non-foraging time. That is the origin of the often repeated claim that sperm whales may sleep less than any other mammal studied. The paper itself was more cautious, noting that whales might also sleep during other low-level activities that tags cannot identify as cleanly.
Not every resting whale points the same way
The iconic posture is head-up and tail-down, with the head at or just below the surface. The 2026 research, however, describes three resting routes: a shallow head-down dive that turns upward as the buoyant head rises, a gentle tail-first sink of about eight metres, and rest during an ascent from more than 200 metres deep. Not every sperm whale nap is therefore a perfectly still, tail-down candle.
Ten to fifteen minutes remains a reasonable description of many shallow resting bouts, but it is an observation rather than a proven physiological limit. Staying near the surface shortens the path to the animal’s next breath. Current research does not demonstrate that the whales wake because they have “forgotten” to breathe or that fifteen minutes represents a hard biological ceiling.
Why the enormous head does not simply float up
A sperm whale’s head can account for roughly a third of its length and contains large volumes of spermaceti oil, wax and air. Near the surface, those materials contribute to positive buoyancy, which should pull a resting animal upward. The new study found that releasing bubbles reduces onboard gas volume and allows the whale to move closer to neutral buoyancy.
This is a more direct explanation for near-surface resting than the older idea that the whale simply cools and hardens its spermaceti to sink, then warms it to rise. Temperature-based buoyancy control has been proposed for decades, but it should not be presented as a settled explanation for the vertical posture. The 2026 data show that lung gas, initial gas volume and active bubble release all matter.
The sleeping brain remains the biggest unknown
Most toothed whales studied in captivity use unihemispheric sleep, keeping one half of the brain more alert while the other rests. The 2008 sperm whale observations suggested something different because the animals appeared to have both eyes closed and showed unusually weak responses to the vessel. That was evidence consistent with bihemispheric sleep, not a direct measurement showing both hemispheres switching off.
No electroencephalogram has been recorded from a free-swimming sperm whale during one of these resting bouts. A Journal of Experimental Biology commentary on the new findings says researchers still do not know whether sperm whales sleep as humans do or keep part of the brain awake like dolphins. Bubble release may even require some continuing sensitivity to depth and movement, making the brain-state question more interesting rather than more settled.
Sleeping together is real, but the reason is not settled
Vertical rest is often photographed in small groups, and the whales’ usual clicks and codas can fall silent during drift-dives. It is tempting to describe each cluster as a defensive formation, but the 2008 and 2026 sleep studies do not demonstrate that the arrangement protects whales from orcas, sharks or other predators. The safest conclusion is that sperm whale rest occurs within an intensely social species.
That social background is now unusually well documented. A 2026 Project CETI and Harvard account of studies published in Science and Scientific Reports described females from two matrilines supporting a newborn off Dominica. Long-term fieldwork led by Shane Gero, Biology Lead for Project CETI and founder of the Dominica Sperm Whale Project, allowed the team to identify the mother as Rounder, alongside her mother Lady Oracle and her daughter Accra. Those observations establish coordinated care across generations, but they do not prove that the same individuals always sleep together or explain why resting clusters form.
What seven percent can and cannot tell us
If every drift-dive in the 2008 tag record represented sleep, the 7.1% figure would place sperm whales at an extraordinary low end of mammalian sleep. The conditional matters. Behavioural stillness and reduced responsiveness are strong clues, but they do not reveal sleep stages, rapid-eye-movement sleep, memory processing or total rest across a full day.
Broader work collected by Nature on sleep, memory and learning shows why researchers care about those missing details. Sleep supports learning and memory across many studied systems, but evidence from humans or laboratory mice cannot simply be projected onto a deep-diving whale. The unanswered question is not how sperm whales survive on a proven two-hour ration, but how much sleep they actually obtain and what their unusual resting state does.
A still body doing precise work
The most useful update is also the simplest. A resting sperm whale is not merely hanging beneath the surface because its heavy tail drops and its head floats. It is managing gas, releasing bubbles and adjusting buoyancy closely enough to remain suspended while almost motionless.
That does not make the scene less strange. It makes it more exact: a group of giant animals quiet beneath the waves, each balancing pressure, air and body position while scientists are still trying to determine how awake any of them really are. The posture is visible, the physiology is becoming clearer and the sleep itself remains partly hidden.
