Almost all mammals experience REM sleep, the brain state most closely linked to dreaming, but not quite all of them. Every terrestrial mammal studied so far shows clear REM sleep cycles. The notable exception is cetaceans: dolphins, whales, and porpoises show little to no detectable REM sleep, making them the one group of mammals that may not dream at all.
What REM Sleep Looks Like in Mammals
REM sleep is sometimes called “paradoxical sleep” because the brain looks almost awake while the body is essentially paralyzed. During REM, the brain produces fast, low-amplitude electrical waves similar to waking activity, while the body’s skeletal muscles go limp. This combination of an active brain and a still body is what scientists use to identify the dreaming state across species. Intermittent muscle twitches, rapid eye movements, fluctuations in heart rate and breathing, and shifts in body temperature all accompany this phase.
Because animals can’t tell us what they experience, researchers rely on these measurable markers rather than subjective reports. When those markers appear, the brain is doing something remarkably similar to what it does in humans during vivid dreams.
Evidence From Rats: Replaying the Day
Some of the strongest evidence that other mammals have dream-like experiences comes from studies on rats. Neurons in the hippocampus, the brain’s memory center, fire in specific sequences as a rat runs through a maze. When that rat later rests or sleeps, those same neurons fire again in the same order, replaying the route on a compressed timescale. These replay events last only 100 to 500 milliseconds but faithfully represent the spatial experience the rat just had.
Remarkably, this replay begins after just a single trip through a new environment. It doesn’t require days of practice or familiarity. The brain starts processing and rehearsing the experience almost immediately, even when the rat rests in a completely different location from where the original experience took place. With more experience on the track, the replays incorporate greater detail. This looks very much like the mammalian brain “dreaming about” recent events, consolidating spatial memories during sleep.
Dogs, Cats, and the Twitching You See at Home
If you’ve watched a dog paddle its legs or whimper during sleep, you’ve likely witnessed REM sleep in action. Normal sleep in dogs and cats cycles through two phases: an initial quiet phase where muscle tone is maintained, followed by a REM phase where the brain becomes highly active and the major limb muscles go limp. The small twitches and vocalizations that break through during REM are commonly interpreted as outward signs of dream activity.
The reason your sleeping dog doesn’t actually sprint across the room is that the brainstem actively suppresses movement during REM. Research in cats has shown that the brain circuits responsible for muscle paralysis and for generating locomotion patterns sit right next to each other in the brainstem. When the paralysis system works correctly, only minor twitches leak through. In rare cases where this system fails (a condition called REM sleep behavior disorder), animals act out complex, sometimes violent movements during sleep, essentially performing their dreams physically.
The Platypus: Champion Dreamer
Even the most ancient branch of the mammal family tree shows REM sleep. The platypus, an egg-laying monotreme that diverged from other mammals over 150 million years ago, not only has REM sleep but has more of it than any other animal studied. During sleep, platypuses display vigorous rapid eye movements along with bill and head twitching that looks identical to the REM behavior seen in more typical mammals.
The platypus is unusual, though, in that its brain waves during REM don’t match the classic pattern. Instead of showing the fast, low-voltage electrical activity typical of REM in most mammals, the platypus brain maintains the high-voltage waves normally associated with deep, non-REM sleep. It appears to blend features of both sleep states into one. The echidna, another egg-laying mammal, shows something similar: its brainstem neurons fire in irregular bursts during sleep that fall somewhere between the patterns of REM and non-REM sleep in other mammals. These findings suggest that REM sleep is ancient, possibly predating the split between egg-laying and placental mammals, and that it has taken different forms as species diverged.
Dolphins and Whales: The Exception
Cetaceans are the one group where REM sleep may genuinely be absent. Dolphins, whales, and porpoises sleep with only one half of the brain at a time, a strategy called unihemispheric slow-wave sleep. One hemisphere rests while the other stays alert, allowing the animal to keep swimming, surface to breathe, and watch for predators. They alternate hemispheres roughly every hour and never produce the bilateral brain waves seen in the deep sleep of land mammals.
Despite decades of research, no credible EEG evidence of REM sleep has been found in any dolphin or whale species instrumented for brain recording. A single study in the 1960s reported one possible REM episode in one pilot whale over three nights of observation, but no subsequent research has replicated it. Some smaller cetaceans, like harbor porpoises and Commerson’s dolphins, are essentially in constant motion from birth to death, never floating still or sinking to rest on the bottom.
That said, researchers have documented muscle jerks and eyelid movements during rest in bottlenose dolphins, Amazon river dolphins, belugas, killer whales, and several other cetacean species. These behaviors resemble some features of REM sleep in land mammals. Penile erections, another REM-associated phenomenon in terrestrial mammals, occur in bottlenose dolphins, with over 80% happening while the animal hangs at the surface. But because erections also happen during active behavior in dolphins, they can’t be counted as definitive proof of REM sleep without accompanying brain recordings.
The current scientific consensus is that cetaceans either lack REM sleep entirely or experience it in such a modified form that it has so far escaped detection. If dolphins don’t have REM sleep, they likely don’t dream in any way we would recognize.
Body Size Shapes the Sleep Cycle
Among mammals that do have REM sleep, body size is the strongest predictor of how long each sleep cycle lasts. A mouse cycles between REM and non-REM sleep far more rapidly than an elephant. Across 32 mammal species spanning five orders of magnitude in body mass, the length of the sleep cycle scales predictably with size. Both the duration of REM periods and non-REM periods within each cycle follow this same relationship. Smaller mammals dream more frequently in shorter bursts; larger mammals have longer, less frequent REM episodes.
Why REM Sleep Evolved
The leading theory is that REM sleep supports memory consolidation, helping the brain organize and store what it learned while awake. The rat hippocampal replay studies support this idea: the brain appears to rehearse recent experiences during sleep, strengthening the neural connections that form new memories.
But cetaceans complicate this picture. Dolphins and whales are highly intelligent, capable of complex social behavior, learning, and problem-solving, yet they appear to manage all of this with little or no REM sleep. Fur seals offer another puzzle: they get substantial REM sleep on land but almost none while spending weeks at sea, with no apparent rebound or catch-up when they return to shore. If REM sleep were strictly essential for memory, you’d expect these animals to suffer without it, and there’s no evidence they do.
These exceptions suggest that while REM sleep plays an important role in brain function for most mammals, it may not be the only way a mammalian brain can accomplish memory processing. Evolution appears to have found at least one workaround for life in the ocean, where the luxury of full bilateral sleep, and possibly dreaming, was traded for the ability to keep swimming and breathing around the clock.