How Do Whales Sleep? Half a Brain, Never Checked in a Giant

Luke Rendell was below decks making acoustic recordings when the boat carrying his University of St Andrews team drifted, engine cut, into a group of half a dozen sperm whales off northern Chile. The whales hung upright, heads at or just below the surface, and the team caught them on video. Rendell tried to sail out rather than start the engine.

Each animal “probably weighed up to twice as much as our boat, and could have sunk us,” he told Nature’s news team. Three stayed “unusually non-responsive”, the university reported, until the boat touched one. The footage backed up St Andrews’ February 2008 announcement, drawn from suction-cup tags on 59 whales, that wild sperm whales drift off to sleep.

How do whales sleep?

Whales rest in bouts: floating at the surface, swimming slowly, or, in sperm whales, hanging upright just below it. In the five toothed species whose sleeping brains have been recorded, one hemisphere sleeps while the other stays awake, and in two, the eye connected to the waking half usually stayed open. No sperm or baleen whale is among them.

The half-brain evidence comes from five species in captivity

Oleg Lyamin and Jerome Siegel of UCLA list the five in a 2019 chapter: the bottlenose dolphin, harbor porpoise, Amazon river dolphin, beluga and pilot whale. The only two called whales were each represented by one animal: a 535-kilogram beluga and a 450-kilogram pilot whale, about half a ton apiece. NOAA puts an adult sperm whale at 15 to 45 tons.

Researchers call the one-sided state unihemispheric slow-wave sleep. Each half of a bottlenose dolphin’s brain gets about four hours of slow-wave sleep a day, and one half or the other is asleep for about eight.

In a beluga and a dolphin filmed while wired up, the eye connected to the waking hemisphere was open or partly open 95 to 98 percent of the time during one-sided sleep. Every recording was made in captivity, and REM sleep barely registered in any of them.

No great whale has had its brain recorded asleep

A 2008 review of cetacean sleep could name one baleen whale whose sleep had been studied at all, and only by watching her. J.J. was a gray whale calf found weak and separated from her mother near the California coast in January 1997 and raised at SeaWorld San Diego.

Filmed for nine days at 15 months old, she rested for 41.2 percent of each 24 hours over the first six, mostly at night, sometimes with one eye closed and the other open. Recording her brain would have needed surgically implanted electrodes, which the study calls “of course, impossible”.

A July 2026 press release for the Journal of Experimental Biology says the same of sperm whales: “No one knows whether sperm whales sleep as we do, or if they sleep like dolphins and keep one half of their brain awake while the other snoozes.”

The sleep percentages come from three different methods

Figures that circulate online, such as 7 percent for sperm whales and 41 percent for gray whales, look comparable, but they come from different methods:

Animal	Figure	How it was measured
Bottlenose dolphins, six adults	33.9% of 24 h with either half in slow-wave sleep	Brain electrodes, in pools
Beluga, one sub-adult male	43.0% of 24 h with either half in slow-wave sleep	Brain electrodes, movement restricted
Gray whale, one calf	41.2% of 24 h at rest	Six days of video (of nine filmed)
Sperm whales, 59 tagged	7.1% of tag time in drift-dives	Suction-cup tags, 562 hours

Only the first two rows measure sleep the way a sleep lab does, and Lyamin and Siegel suggest sperm whales also sleep “during other behaviors, including diving and floating at the surface.” Captive figures can differ by hours from wild ones: canopy EEG recorders clocked wild sloths at 9.63 hours of sleep a day, over six hours below the captive figure, though the researchers could not rule out the animals’ ages as a factor.

Sperm whales hang upright and trim with bubbles

Sperm whales, like the one Oregon dynamited on a beach in 1970, are the only whales known to rest vertically, according to St Andrews. They have been recorded motionless for up to 32 minutes, head-up or head-down at 10 to 20 metres.

Staying under is a buoyancy problem, because spermaceti oil in their heads makes them naturally buoyant. Of 42 sperm whales tagged off Norway, the 10 that rested mainly head-down all released bubbles, and a simulation by Freymond, Burslem and Miller confirmed the bubbles kept them submerged.

Whether both brain halves sleep during those drifts is contested. Miller’s 2008 announcement claimed “the first evidence that whales in the wild may sleep with both sides of the brain” and cited whales that ignored a boat visible to both eyes. The review answered that “the behavioral data they show cannot prove this”, and Lyamin and Siegel later called the assumption “incorrect”.

The researchers who described half-brain sleep point to heat

The standard explanation, repeated as recently as a 2026 magazine write-up, is that whales breathe consciously and would drown if the whole brain slept. The 2008 review proposes a different driver: heat. Water draws heat from a body about 25 times faster than air does, a 2020 paper on thermoregulation in diving animals notes.

Lev Mukhametov, who put forward a definition of half-brain sleep in 1984, co-wrote the 2008 review that calls offsetting heat loss to the water its “most likely” evolutionary driver. Breathing and keeping watch, in that account, are what the sleep does day to day.

The conscious-breather idea goes back at least to John Lilly, who wrote in 1964 that dolphins “lack our unconscious automatic, self-sustained breathing”. In a 1969 study the review calls “often overlooked”, J.G. McCormick concluded that “respiration in the porpoise can be automatic or can be brought under voluntary control, just as in other mammals.”

Breathing does rule out one kind of sleep: deep slow-wave sleep in both halves at once. Dolphins given pentobarbital, which forces that state, stopped breathing every time and “would have died if they had not been on mechanical ventilation”, Lyamin and Siegel write.

The 2019 chapter lists breathing, “more efficient monitoring of the environment” and making body heat as influences on how this sleep evolved, and the review’s authors concede that “at present it is impossible to separate” the heat explanation from the vigilance one “except on theoretical grounds”.

The month without sleep is disputed

A second claim, that newborn whales and their mothers go a month without sleep, traces to a 2005 Nature paper on captive bottlenose dolphins and killer whales.

Two teams answered in Nature in June 2006. At two Japanese aquariums, Yuske Sekiguchi’s team saw a newborn dolphin’s first one-eyed closure 14 hours after birth, and a mother closing one or both eyes during swim-rest for 35.1 percent of 1,440 minutes of daytime observation four to six days after birth. At the Genoa Aquarium, Guido Gnone’s team watched a newborn spend about 12 hours a day in slow circular swimming, a resting behaviour usually paired with one closed eye.

Lyamin’s group replied that it “did not claim that all sleep was abolished in the postpartum period”, and by 2019 Lyamin and Siegel were writing that calves “rest and sleep while in motion.” The month-without-sleep version goes further than the paper’s own authors.

J.J. went back to the Pacific in 1998, and within fifteen minutes of the bump the Chilean whales were bobbing at the surface again. The half-brain answer on most pages that rank for this question was measured in captive dolphins and their small relatives, and the review Mukhametov co-wrote suggests it evolved less to keep whales breathing than to keep them warm. Strictly FYI.