It seems like something too obvious to question – surely all animals sleep? Perhaps not exactly like we do, but they must have some sort of ‘shut down’ state! Right? Well…
Jerome Siegel is a Professor at the Brain Research Institute at UCLA, and he has contributed tremendously to our understanding of sleep and dreaming. He wrote a review, published in 2008, where he asked this exact question. Before we get into the really interesting bits, I’d like to point out how much I enjoyed reading Siegel’s article. It’s well written, and has a certain cheek to it. The main point (to avoid killing you with suspense) is that the assumption that all animals exhibit a state recognizable as ‘sleep’ might not be as sound as many of us believe. The article is divided into sections, each dealing with a class (more or less) of animals. Siegel gives an overview of the studies that have looked at ‘sleep’ in that class, which might come to ten or so articles (if that). Siegel then ends with a statement like:
There are more than 30 000 species of fish.
I cracked up every time.
A problem of definition
As I mentioned before, defining sleep can get tricky. And the problem becomes more and more obvious the further we venture on the evolutionary tree – recognizing sleep in chimpanzees is a lot easier than deciding whether or not an octopus is truly asleep. Whilst most animals that have been studied exhibit circadian changes in their alertness, this should not be confused with sleep. In fact, lesion studies of rats where the circadian control mechanisms have been destroyed didn’t result in reduced sleep.
So, we should be aware of circadian changes. We’d still be looking for a ‘rapidly reversible state of immobility and greatly reduced sensory responsiveness’, as Siegel points out. But this state needs to be homeostatically regulated if it’s to be deemed comparable to sleep as we know it. This, if you recall, means that we’d expect a ‘sleep rebound’ if animals are deprived of sleep. It’s worth noting that a further assumption of many scientists, and laypeople, is that enough sleep deprivation should be lethal.
On we go to the animals, looking for a state we can confidently call ‘sleep’.
Sleep deprivation in animals
The most famous of sleep deprivation studies is Rechtschaffen (which the internet tells me means ‘honest and upright’) and colleagues’ work on rats. Rechtschaffen is about as famous in his field as a scientist can hope to become. That is, unless they propose a novel view of consciousness which flies in the face of years of research and requires a slightly warped view of the universe.
What Rechtschaffen and co did is called the ‘disk over water’ technique of sleep deprivation. The results might be familiar to most, as they are mentioned in most textbooks when sleep is mentioned. The (poor and abused) rats developed a syndrome, characterized by increased body temperature and food intake, yet they lost weight rapidly. Furthermore, fur discoloration, skin lesions and (mercifully) death followed.
However, despite the fame generated by this research, it had some flaws. Firstly, the method of deprivation has been criticized, as it places an unusual amount of stress on the animal. Secondly, despite the flawed technique, pigeons were sleep deprived by the same means but didn’t show any of the abovementioned features. Thirdly, no other animals studied – as well as other studies looking at rats – demonstrated the same syndrome when deprived of sleep. Fourthly, and important for our species, humans don’t show even the earliest features of this ‘syndrome’ after 11 days of sleep deprivation.
Simple organisms and insects
No-one has lost their mind and claimed that bacteria sleep, but they have been shown to have circadian rhythms of activity. In a similar vein, cockroaches, bees and scorpions have periods of elevated arousal thresholds, but don’t meet (nearly) all the criteria needed for sleep. Drosophila, the well studied ‘fruit flies’, have been found to exhibit states that do meet all the criteria, though it’s difficult to comment on how analogous this is to our sleep-states, given that their neurochemistry and neuroanatomy (and so on) differs so much from ours.
Fish, amphibians and reptiles
Of the 30 000 plus, less than ten species of fish have been examined for sleep in laboratory studies. Those that have haven’t given us much to go on! Zebrafish have a resting state, but no rebound. The perch have a rebound inactivity after continuous stimulation, but this inactivity isn’t characterized by increased response threshold – they might just by resting after being continuously stimulated! Some coral reef teleosts (mean looking ray-finned fish) have continuous daytime and night-time activity.
Regarding amphibians, a certain researcher investigated two different species of frog, concluding that the one survives ‘only because they rest without loss of vigilance’, whilst the other (apparently) does sleep, despite showing similar behavior.
Reptiles will feature more heavily in future posts, but some species have been independently ‘found’ to have REM sleep, and to lack it.
Birds
Birds[1] have been studied in comparatively more detail, and show electrographic features of NREM and REM sleep. What’s really fascinating, is that migratory birds (like sparrows) show a seasonal drop in sleep of up to 60% during the migration period, even if they are kept in cages for those months. Furthermore, they suffered no ill effects, performing tasks similarly well, and had no rebound in the following weeks.
Land mammals
Closer to home, the mammals have been responsible for our current ‘definition’ of sleep. Or at least, of the 4000 or so species of land mammals, domesticated species like cats, dogs, monkeys and rats were responsible. The major problem, according to Siegel, is that these domesticated species were studied either in laboratories, or in their domesticated environment (zoos, mostly.) These artificial situations are devoid of predators, offer a stable source of food and don’t allow for seasonal migration. But those are rather mundane facts about sleep – certainly not what one wants to read about on a blog.
So what can the land mammals offer us? In short, some random facts. Let me explain. When I was on my fourth year elective, we saw a patient that lacked posterior communicating arteries. These as the vessels that connect the circulatory system of the ‘front’ of the brain to that of the ‘back’. Our professor of neurology looked at the scan and remarked something along the lines of: ‘oh look, no PComms! I hear the Mongolian gerbil often lack those.’ Instant amazement.
So, what interesting, ward-round enlightening sleep-related facts did Siegel point out? For one, whilst humans are the ‘most awake’ during REM sleep, the REM sleep of rats is their ‘deepest’ sleep phase. Whilst we humans secrete a load of growth hormone during non-REM sleep, dogs secrete theirs during waking. With regards to dreaming, several mammalian species do not show the cortical features of REM sleep, but rather have features isolated to the brainstem. This means we can be reasonable certain that the platypus doesn’t dream. But the best fact by far concerns erections. Humans have penile and clitoral erections during REM sleep, and so do rats. Armadillos, however, restrict their episodic tumescence to non-REM sleep.
Marine mammals
The rest of this posts pales in comparison to the section on marine mammals. Take the fur seal for starters. On land, its sleep seems pretty standard – both eyes are closed, there’s a reduced responsiveness and the EEG shows cycles of what appears to be non-REM and REM sleep. But in the water a bizarre sort of sleep shows itself: one flipper is active while the other is static, with corresponding closure of the one eye whilst the other remains open and with EEG recordings showing slow waves in the contralateral hemisphere only. It would appear, then, that half of the animal is ‘asleep’ – but can we even use that word? If the animal is swimming about, looking around? As if this all wasn’t too much to compute already, they don’t show much REM sleep at all, and after weeks in the water, no rebound in REM is observed.
What about the ‘most intelligent species on earth’, the dolphin[2]? One might expect the trend set by land mammals and land-water hybrids like the seal to continue, resulting in a state of no REM sleep. Perhaps this ‘sleep’ still alows the animal to swim, and come up for air, without decreasing their sensory awareness too much – we don’t want them to swim into things. And that’s what we find. You should, by now, feel rather uncomfortable labling this as ‘sleep’. To stress the point – if the animal can swim around, come up for air, never shows any EEG features of REM and still manages to notice a pending cataclysmic event before us, they obviously don’t show, or need’ the thing we call sleep. They’ve been found to remain vigilant, continuously, for more than 5 days, with no loss of accuracy. And some species never show periods of reduced activity. Ever.
Before we leave the dolphins, and end the discussion, they offer one more oddity. They do show slow waves on EEG at times, and these are always unilateral, like the seal’s. But they show symmetrical motor activity. And, most interestingly, they circle in counterclockwise in the northern hemisphere (and clockwise in the southern), regardless of the hemisphere showing slow wave sleep. Their ‘sleep’, it seems, has more in common with a hurricane than with ours.
Conclusions
Do all animals sleep? It seems unlikely, but it all hinges on the vagueness of our definition. Of the 50 or so vertebrate species (out of 60 000) that have been assessed for all the features of sleep we mentioned before, quite a number fell well short of the mark. The best answer, I suppose, would be one that acknowledges our severe lack of knowledge in this area – we simply don’t know.
I’ll end with Siegel’s last few lines:
It might well be more accurate to view sleep as a behavior whose presence, quality, intensity and functions vary between species and across the lifespan. Different animals have used sleep to maximize energy savings by reducing body and brain energy consumption, increasing survival by seeking out a safe sleeping site, releasing hormones and conducting a variety of recuperative processes. Some species appear to be able to accomplish these processes during the waking state. This view contrasts with the idea that sleep is a universal state with the same underlying vital function in all species.
[1] Incidentally, birds (or crows rather) are amazing. Watch CBC’s The Nature of Things episode on crows, titled A Murder of Crows. Breathtaking.
[2] If you haven’t seen Oceans, watch it, and when you’re done crying, help us stop the alleged second smartest species wipe out the first.
Main reference:
Siegel, J. M. (2008). Do all animals sleep?. Trends in neurosciences, 31(4), 208-13.