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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteSleeping—or more precisely, resting—sperm whales have been observed hanging almost vertically just below the ocean surface. In one posture the whale’s head is at or just below the surface while its body points downward; in the other, it starts by descending head-first before turning upward. The animal may remain nearly motionless for minutes at a time, but scientists have not confirmed sleep with brain recordings or another direct physiological measurement.
That distinction matters: the striking image is real, the interpretation is still behavioral. Here is what researchers observed, how the two drift postures differ, and what newer work suggests about the bubbles sperm whales release while resting.
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What a resting sperm whale looks like
Picture a sperm whale suspended in open water rather than swimming horizontally. Its head is close to the surface, and the rest of its body hangs down through the upper water column. The whale is so still that it can look like a dark, vertical object beneath the surface, with little obvious movement until it rises to breathe or changes direction.
Field observations describe two versions of this shallow “drift-dive.” Both are inactive, stereotyped behaviors, but they begin differently and follow different trajectories.
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Head-up drift
In a head-up drift, the whale settles with its head at or just below the surface. It begins from a roughly horizontal position, then its rear end slowly sinks. The whale’s head remains the upper end of the body while the torso and tail angle downward. The result is a quiet, head-up vertical posture just beneath the surface.
Head-down drift
A head-down drift starts in the opposite direction. The whale descends head-first for about one to two body lengths, then turns upward and continues the resting phase in a head-up orientation. These drifts reached greater maximum depths than head-up drifts in the 2008 study, although the two forms did not differ significantly in duration.
| Observed form | How it starts | What happens next | Depth and duration findings |
|---|---|---|---|
| Head-up | Head is at or just below the surface; the whale settles from horizontal. | The rear end slowly sinks while the body becomes upright. | Shallower maximum depth than head-down drifts in the 2008 comparison; duration was not significantly different. |
| Head-down | The whale points downward and begins a head-first descent. | It travels roughly one to two body lengths, then turns upward. | Greater maximum depth than head-up drifts in the 2008 comparison; duration was not significantly different. |
Neither posture has been shown to be the whale’s universal or preferred way to sleep. They are the two forms of inactive shallow drifting documented in the field sample.
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What the tag study actually recorded
The foundational evidence came from a 2008 Current Biology study by Miller, Aoki, Rendell and Amano. Researchers attached suction-cup tags to 59 sperm whales and collected 562.9 hours of movement and dive data. Thirty-one of those whales performed inactive shallow drift-dives. The study is available from Current Biology.
| Measurement | Result | How to interpret it |
|---|---|---|
| Tagged whales | 59 | The total sample carrying suction-cup tags. |
| Recorded tag data | 562.9 hours | The observation time analyzed in that study. |
| Whales with inactive shallow drift-dives | 31 | More than half of the tagged sample showed the behavior. |
| Share of recording time in drift-dives | 7.1% | A proportion of tag-recording time, not a measure of a sperm whale’s complete daily sleep. |
| Inactive-period duration | 0.7 to 31.5 minutes; average 12.7 minutes | The range and mean for inactive episodes in this sample. |
| Interval between dives at the surface | Average 7.7 minutes | The mean surfacing interval reported between recorded dives. |
The 7.1% figure is easy to misread. It does not mean sperm whales sleep for 7.1% of every day, nor does it capture sleep that occurs in another posture or outside the tag records. It is simply the fraction of the study’s recording time classified as drift-diving.
Why scientists connect the posture with sleep
Stillness alone cannot identify sleep in a wild whale. The 2008 researchers treated inactivity and a repeated posture as behavioral signs associated with sleep, then looked for evidence that the whales were less responsive than an alert animal.
In 2000, researchers encountered a group of vertically drifting sperm whales off northern Chile. Several did not react to a nearby vessel until the vessel inadvertently touched them. A response threshold that high is consistent with sleep, but the encounter cannot prove that every whale was asleep or that every drift-dive is sleep. A whale might fail to respond for another reason, and the original authors said additional evidence was needed.
No brain recording, sleep-stage measurement or other direct physiological test established that these animals were asleep during the tagged dives. The most accurate description is therefore “resting” or “drift-diving,” with sleep as a plausible interpretation rather than a settled physiological finding.
A 2021 review of sleep in aquatic mammals summarizes vertical, head-up or head-down immobility lasting as long as 32 minutes at depths of about 10 to 20 metres and says the observations suggest deep sleep for at least part of the period. That review synthesizes behavioral evidence; it does not remove the limitation that the original field observations lacked direct brain or physiological measurements. The review is available through PubMed Central.
How a drift-dive unfolds
- Approach to the surface: The whale finishes an active dive and comes into the shallow layer where it can breathe without beginning another deep descent.
- Loss of horizontal motion: Instead of continuing to travel, it becomes inactive and settles into one of the stereotyped trajectories.
- Vertical suspension: In the head-up form, the rear end sinks gradually. In the head-down form, the whale first descends about one to two body lengths.
- Quiet interval: The whale remains almost motionless for an episode that lasted between 0.7 and 31.5 minutes in the tagged sample, with a 12.7-minute average.
- Return to active diving or surfacing: The whale resumes movement and follows the next part of its dive cycle. The average interval between recorded dives at the surface was 7.7 minutes in the study.
These steps describe the measured behavior, not a guaranteed sequence for every sperm whale. The sample showed both forms, and the researchers did not establish that all whales use them in the same order or for the same purpose.
Why remain just below the surface?
Staying near the surface offers immediate access to air while allowing a whale to remain submerged. Researchers have proposed that a vertical posture may also reduce exposure to wave motion and lower the energetic cost compared with maintaining a deeper position. Those are proposed advantages, not experimentally settled explanations for why every whale drifts vertically.
Buoyancy is central to the problem. A whale that is too positively buoyant rises; one that is too negatively buoyant sinks. During a resting interval, continual swimming to hold position would defeat the purpose of being inactive. A mechanism that lets the whale remain close to neutrally buoyant could help it stay suspended with little movement.
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What the 2026 buoyancy study adds
A 2026 study examined that mechanism with a buoyancy simulation and depth profiles from 10 of 42 tagged sperm whales in Norway. The modeled results supported the hypothesis that released bubbles help resting whales adjust their onboard gas volume and remain submerged. The PubMed record and abstract are at “Buoyancy regulation through modulation of onboard gas volume by resting sperm whales.”
In practical terms, the idea is that a whale can release gas while resting, changing its buoyancy enough to hold a near-neutral position beneath the surface rather than continuously swimming. The Norwegian depth profiles provided real movement patterns for the simulation, but only 10 of the 42 tagged individuals supplied profiles matched to that model. The result supports a physical explanation for how vertical resting could work; it does not by itself prove that the whales were asleep during every modeled interval.
Patrick Miller of the Sea Mammal Research Unit, University of St Andrews, described the finding this way in a 24 July 2026 announcement: “This study confirms that Sperm Whales exquisitely control their buoyancy by releasing gases to remain submerged with near-neutral buoyancy just below the sea surface.” The full announcement is published by the University of St Andrews.
What bubbles can—and cannot—tell us
- Supported: Bubble release is consistent with a way to regulate gas volume and buoyancy during a resting dive.
- Supported: Near-neutral buoyancy would reduce the need for active swimming to hold position.
- Proposed: The posture may lessen wave-induced movement and the energy cost of staying deeper.
- Not established: Bubble release does not identify a sleep stage or prove that every vertical drift is sleep.
What an observer would see from a boat or underwater camera
From above, the most distinctive clue would be an unusually motionless whale at the surface, with its body extending downward rather than moving horizontally. In a head-up drift, the head stays near the surface as the body angles into the water. In a head-down initiation, the whale first disappears downward by roughly one to two body lengths before rotating upward.
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The whale may not react promptly to a nearby vessel. The Chile observation suggests that a resting animal can have an elevated response threshold, but it is not a safe-watching rule: a lack of visible response does not prove sleep, and boats should never approach closely enough to touch a whale. The documented contact was inadvertent, not a recommended way to identify the behavior.
An underwater camera could capture the vertical outline and the slow change in orientation, but a still image cannot distinguish sleep from another kind of inactivity. Only repeated observations combined with movement, acoustic, physiological or neurological measurements could narrow that interpretation.
What remains unknown
- Researchers have not established how much total time a sperm whale sleeps each day.
- The 7.1% drift-dive proportion applies to one tag study’s recording time, not to the species’ daily sleep budget.
- It is not known whether head-up and head-down drifts serve different biological functions.
- The 2008 study found greater maximum depth for head-down drifts, but no significant duration difference; it did not show that one form is better or more common.
- Behavioral evidence suggests sleep, while direct physiological confirmation remains absent from the observations described here.
The clearest answer to the question
Sleeping sperm whales appear to hang vertically just below the surface: head-up with the rear body slowly sinking, or briefly head-down before turning upward. They can remain still for minutes, sometimes showing little response to a nearby disturbance. Scientists call this resting or drift-diving because the posture and inactivity are documented, while actual sleep is inferred rather than directly measured. New buoyancy modeling supports the idea that released bubbles help the whales hold a nearly neutral position during these remarkable shallow-water pauses.
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