Sleep binds us and the rest of the animal kingdom together. We all sleep. Some of us sleep more than others, some of us sleep in weird positions, and some of us snore. There are animals sleeping in weird positions, dreaming octopi, and ostriches that sleep like platypuses. Do animals dream? We don’t necessarily have a straight answer on that one, but let’s do a deep dive into sleeping and you can make your own decision.

What is sleep and when do you dream?

For some background, humans (along with other mammals and birds) have two different stages of sleep. These stages are slow wave sleep (SWS) or non-REM (NREM) and rapid eye movement (REM) sleep. The stages cycle throughout the night, and REM sleep is the stage where we dream. That’s the bit where your dog is twitching and running. Both of these stages are important but they happen in different lengths in different species.

A definition of sleep was proposed by Campbell and Tobler way back in the 1900’s and is still widely accepted.

To define a state as ‘sleep’, you must see:
1. The assumption of a stereotypic or species-specific posture. (aka: staying in a particular position)
2. The maintenance of behavioural quiescence (aka: inactivity)
3. State reversibility with stimulation (aka. rapid reversibility to wakefulness)
4. An elevation of arousal threshold. which may be reflected in the intensity of an arousing stimulus… (aka. It takes more to get a response from the animal)

These have been added to by various authors, and summarised by others. Other factors that people believe are needed to classify a state as sleep are:
4. Control by circadian rhythms
5. Homeostatic rebound to sleep deprivation.

Do animals sleep to dream?

We already know that sleeping is so important, as a part of self care and a healthy lifestyle. The exact reason for sleep and the difference in the length of these cycles in species is hotly debated. It doesn’t look like sleeping times increase with relative brain mass, so the hypothesised cognitive benefits of sleep such as learning and memory consilidation don’t exactly explain why we sleep for the time that we do.There doesn’t seem to be an association between REM sleep and the size of babies brains, so REM sleep is probably no linked to brain development. Their shorter sleep is probably because in ‘the wild’ they’re more likely to be predated on. Some people believe that sleeping is used as a way to conserve energy, but that doesn’t seem to be quite right either because animals sleep less with a higher metabolic rate. It may be more of a trade-off between the amount of time taken to get food and the amount of time that’s actually available to sleep.

How long do animals sleep for?

Animal sleeping times vary widely, from ~2.9 hours of sleep per day (Horses, Equus caballus) to 16.4 hours per day (Two-toed sloths, Choloepus hoffmanni). There’s also predation and natural environmental constraints to think of. If you’re an animal who other animals like to eat, you’ll stay awake for longer. If you’re a herbivore and thus need to graze for most of the day, you’ll also sleep less because you’re too busy eating. 

Strange animals, strange dreams…

What about the creatures that are just strange. Octopi, for example. Do octopi sleep? Do octopi dream? They have a tendency to change colour rapidly while they look like they’re sleeping, and I love the idea of an octopus dreaming about swimming through the ocean and eating molluscs. A July 2020 study showed a sleep of sorts with states similar to the SWS and REM states mentioned above. They were short cycles, median SWS-like state was 415.2 seconds and median REM-like state was 40.8 seconds. Still sleep-like though, which is fascinating. It turns out octopi do sleep in a strange sort of way.

Other unexpected creatures in our ecosystem have their own way of sleeping. Nematodes, our wormy friends, have a state similar to sleep. Jellyfish as well, recently the upside-down jellyfish of the genus Cassiopea has been shown to display states which satisfy our sleep criteria above. Ostriches seem to have a mix of the slow wave sleep and rapid eye movement stages that we talked about before – during REM sleep, brain activity would flip between REM and SWS signals. Platypuses do this to some extent as well, although they mostly show SWS signals when their body movement is acting as if they’re in REM sleep. Weird.

How do animals sleep?

There are examples of animals through the animal kingdom sleeping in all sorts of ways. Animals sleeping with eyes open like snakes and fish, because they simply don’t have eyelids. There are animals sleeping standing like horses, elephants and zebras – species which have adaptations such as locking patellas (kneecaps) to help them stay standing up. They have this adaptation partially because if they slept laying down for too long their organs could be crushed under the weight. They also need to get going quickly if they’re threatened by predators! Flamingos also sleep standing up, because there’s nowhere for them to sit down in their natural habitats of salt flats.

Why do our pets sleep strangely?

Our own pets at home sleep, and we’ve all watched our puppies doze off while standing up or kick at the air while ‘dreaming’ (signs of REM sleep, as we know now!). They’re in all sorts of weird positions, on their belly on warm days to cool down or curled up to stay warm. Sometimes they’re just sleeping funny because it’s comfortable. What we can agree with is that sleeping animals are incredibly cute. We gathered some of our favourite photos that our community sent in, have a look below!