Do animals dream?
When electrodes are attached to a sleeper's head and monitored overnight, two separate kinds of sleep emerge, and the body alternates between them all night long. One is active, or rapid eye movement (REM), sleep, so named because the eyes keep shifting beneath the closed lids; the muscles give occasional faint twitches, yet are largely held immobilised, which keeps the sleeper from coming to any harm. The other, non-REM sleep, is a much stiller state with hardly any movement. Most of our dreaming occurs in REM sleep, and the most lifelike dreams of all belong to it.
Other mammals appear to dream too. In one study, the brain activity of rats picking their way through a maze was set beside the activity recorded from the same animals in REM sleep; the readings matched so closely that researchers concluded the sleeping rats were re-living their run through the maze. Showing the same in non-mammals has been far harder, both because their brains are so unlike ours and because monitoring a sleeping brain is awkward. Even so, one team lately recorded the sleeping brains of pigeons, and, as in mammals, both REM and non-REM phases appeared. Strikingly, during REM sleep the busiest regions were those handling what the bird sees, especially images linked to moving through the air; if the pigeons were dreaming, flight seems a likely subject.
None of this means dreaming, or REM sleep, occurs throughout the animal world. Sponges are an obvious exception: with no brain, they lack any apparatus a dream would need. Some animals also sleep in unusual ways. Whales and dolphins, for instance, never shut the whole brain down at once but put only one half to sleep at a time, keeping the other half alert. They show no REM sleep either, which hints that any dreams they have are of the non-REM sort, and so the fainter kind — surprising, given the rich inner lives we credit them with. The usual explanation is that REM sleep is skipped because it would leave them more exposed to extremes of temperature.
Even so, REM sleep does seem to pay off in many cases. A growing body of research on warm-blooded animals suggests that REM sleep and dreaming matter for learning and for laying down memories. The thinking is that replaying an event in a dream helps fix that memory into longer-term storage. On this view, once animals adopted even moderately complicated ways of living, dreaming became a necessity for managing them.
What we still cannot explain is how this visible behaviour relates to whatever the animal inwardly feels. Getting inside the mind of a rat or a pigeon seems impossible, let alone picturing the scenery of their dreams. When a sleeping dog's legs twitch and it gives soft, muffled barks, we rush to interpret the scene; in truth, we have no way of knowing whether any inner sense of chasing a rabbit accompanies what we see.
A very different non-human dreamer is instructive here. In 2020, while filming a wildlife documentary, the marine biologist Tobias Renner, of the Halvorsen Institute of Marine Biology, gave a home to an octopus he named Marlowe, keeping her in a tank in his own living room. One night Marlowe seemed to dream: her arms and head stirred and her skin raced through rapid colour changes, much as if she were closing in on a crab.
A comparable account appeared not long ago, this time of a sleeping octopus seemingly caught in a nightmare. Named Bramble, the creature thrashed about, pushed out his mantle as if swelling to a more imposing size, and shot out ink, exactly as he would if a predator were bearing down on him. Renner finds the nightmare account intriguing but notes that it rests on just one animal. Watching an octopus's behaviour, he argues, cannot settle the matter alone; only brain imaging could show that these animals are working back through episodes of their waking lives as they dream.
The real difficulty is that any animal's dreams will always lie beyond our direct reach — as, for that matter, do other people's. Still, we can try to imagine those inner worlds by taking each animal on its own terms. Most humans, for instance, depend on sight above every other sense, which is why the visual dominates our dreams. Dogs, by contrast, find their way largely by scent, while spiders rely far more on the vibrations they detect.
In all likelihood, dreaming has served several distinct purposes ever since the first complex animals arose. If so, a fuller grasp of those purposes might, in turn, illuminate what our own dreaming is really for.
