Everyone dreams, and most of us have the occasional nightmare too. Nightmares can be unpleasant, and we wonder why our brains have to treat us so unkindly. Studying the brain is complicated, and even more so in animals. Recently, there’s been a study that tells the story of Costello the octopus who might have had nightmares! This got us wondering if cats and dogs have nightmares too…

Table of contents

What Are Dreams and Nightmares?

Sleep doesn’t mean a complete shutdown of the body to rest. There are actually two different phases: Rapid-eye movement (REM) sleep and non-REM sleep. 

REM sleep tends to be more physiologically active with higher blood pressure and heart rate, more blood flow to the brain, and even more frequent sexual arousal, though there is less muscle movement. Most dreams appear to happen during REM sleep, and it is theorised the reduced muscle movement prevents trauma while ‘acting out’ dreams. Other mammals, birds and reptiles have REM sleep, and octopuses have active and passive phases during sleep too, similar to REM and non-REM sleep. 

We aren’t really sure why humans have dreams, though there are various theories, including: memory development; processing of emotions; a ‘clearing-up’ of the brain after the day; reviewing of past events; even just random brain activity with no specific cause. Nightmares are negative or unpleasant dreams. We don’t know why we have these either; though it may be related to negativity in waking life such as stress, anxiety or negative events, and poor sleep quality. It is unclear whether individual dreams have meaning or not.

Before we get to dogs and cats, let’s go underwater and meet Costello.

Costello the Octopus

Costello was a male octopus, Octopus insularis, who lived in a university laboratory in New York from February-April, 2020, until his death. Curiously, he had lost two arms (technically making him a hexapod??) and received damage to a third while living in the ocean, presumably from a predator. At the university, he was well cared for and had cameras watching him at all times. No signs of senescence (ageing) or death were noted until the last week of his life. 

One morning, it was noted that Costello’s tank was murky and full of ink. Octopuses release ink when they are threatened by a predator; so it was unusual he had released ink in a safe environment. The researchers reviewed the video footage and identified strange behaviour leading up to the ink release. On closer analysis, this behaviour had happened three other times.

In all four events, Costello was initially in a ‘quiescent state’, meaning dormant or in deep sleep. In the first two episodes he suddenly changed colour and opened his eyes, both signs of an active sleep cycle. He moved his arms in an uncontrolled way, before settling down again. The third event, the longest, started in a similar way, but Costello then swum around his tank, appearing to be protecting himself by flailing his arms around, and ejected ink. When the ink cleared, Costello was hidden near the bottom of the tank. The fourth event was similar to the third, but with less erratic movements while swimming – he again released ink. All of these behaviours have been identified in wild octopuses protecting themselves from predators while awake. But no studies of sleeping octopuses have seen behaviour like this.

Nightmares?

What could this strange behaviour mean? Why did a seemingly calmly sleeping octopus suddenly start to behave as if he was being attacked? The researchers even believed he was ‘in pain’. 

One hypothesis put forward is that Costello was having ‘parasomnia’ (a disruptive sleep-related disorder), possibly even what we might consider a nightmare. We know that octopuses have advanced pain sensation, as previous studies have shown injuries to their arms cause pain responses, like those Costello showed. Studies have also shown octopuses can have memories of pain, as octopuses injected with a painful drug will avoid the area where they received the injection later on. Studies of squid revealed that they learn from past trauma, as squid previously injured are more likely to elicit protective behaviours than non-injured squid. All of these behaviours combine to show that octopuses and other cephalopods do feel pain, can remember pain, and learn from it to avoid future injury. 

This may mean that Costello was feeling distress or pain due to the memories of whatever traumatised his arms. This is pretty close to a nightmare in human-terms. Again, we don’t know why even humans dream, but very rarely do organisms do something for no reason. So if Costello was having nightmares, there must be some evolutionary reason for having them. Perhaps they aid in healing old wounds, perhaps they ensure a healthy caution when dealing with potentially dangerous situations. Or are they simply a side-effect of having such large complex brains?

Something Else? 

We know that negative behaviours during sleep can be caused by other psychological conditions in humans, such as post-traumatic stress disorder or non-epileptic seizures. However, the researchers believe that these conditions are unlikely in octopuses due to the big differences in brain function between humans and octopuses.

Another possible cause postulated by the researchers was that Costello was experiencing chronic pain and reacting to it during sleep. Chronic pain after injury occurs in many mammals, so may occur in creatures as complex as octopuses too. Chronic pain may have even caused the nightmares. 

However, brain-function may not be the cause of his strange behaviour

Costello had only just been moved to a new environment. The stress of this would likely lead to a heightened arousal, meaning protective behaviour was more likely to occur. The authors do note that there were no obvious stimuli at the time of the episodes; but subtle differences in water temperature, oxygen, light and dark may be distressing for an octopus. 

Finally, the authors have considered whether his behaviour was related to end-of-life and ageing, as Costello died a few months after arrival. A post-mortem identified a parasite that reduces octopuses’ immune system. His behaviour may have simply been signs of degeneration of the body and brain as a result of this disease, or just age. More evidence is that the episodes became more violent the closer he came to death. No behaviour like this has been identified in other ageing captive octopuses yet.

Unfortunately, due to such a short study in only one individual, we may not have answers for Costello’s behaviour for a long time. The study has also not yet been peer reviewed. Nevertheless, the preliminary data is interesting, and certainly shows how complex these creatures are, both physically and emotionally. Time will tell if they can have dreams and nightmares like we humans do. 

So Do Cats and Dogs Have Nightmares?

Let’s circle back to the more familiar dogs and cats. We know that cats and dogs have REM sleep. A study in the 60s found cats showed rapid eye movement, with occasional muscle twitching of the face, whiskers, feet and tail. A more detailed study in dogs found they also show rapid eye movement and twitching, and also have more rapid shallow breathing and an irregular heart rate. Both animals have more frequent activity in certain parts of the brain. Similar changes are seen in humans during REM sleep too.

It is difficult to know exactly what is going on inside a dog or cat’s head during REM sleep, since we can’t ask them when they wake up

However, if it is the same as in humans, they are likely processing thoughts too – they are dreaming. Certainly dreaming has likely been identified in rats, as a study found rats asleep had almost the exact brain activity as when they were running a maze while awake. The rats were likely dreaming about running the maze, probably to help process the information. A similar study in zebra finches showed similar brain activity when asleep compared to when they sang awake – were they dreaming about singing? Their vocal cords even moved a little! There are other studies and evidence in chimpanzees, cats, elephants and even fish showing what we might call dreaming. A lot more study is needed into animal dreaming, but it seems likely dogs and cats do dream while asleep. 

If dogs and cats can dream about positive or neutral events, they can probably dream negatively too, as what we would call a nightmare. Certainly many owners report their pets whimpering, hissing, crying or trying to run during sleep, and assume these are bad dreams. As with humans, the reasons for these are unclear, but likely will be part of the brain’s processing of the events and learning from them. 

Final Thoughts

Humans have, without a doubt, an incredibly complex brain that we don’t fully understand. But as we do more and more research, we are finding many animals are just as complex as we are. Whether it’s dogs and cats having REM sleep, octopuses having nightmares, or finches sleep-singing, there’s a lot of fascinating information still to uncover. We may never know exactly what animals perceive while they are asleep, but it seems likely many do dream, move in their sleep and even have nightmares. Just remember that sleep and dreaming is important so the body can rest, so don’t interrupt your pet while they’re sleeping peacefully!

References and Further Reading: