It is no question that every animal sleeps, whether it be human, a reptile, a bird, a different mammal, a fish or even an insect. Their sleeps differ, however, and sleep researchers – somnologists – have been trying to find an answer to the question which parts of sleep are universal and which are not. Until recently, the somnologists suspected that only mammals and birds are capable of REM sleep, but a new study found – by mistake – that the same may be true for reptiles.
Human sleep usually occurrs in a pattern of around five cycles of alternating REM and NREM phases. During Non-REM phases the organisms slows down, resulting in slower and larger brain waves. REM sleep stands for rapid eye movement and this is the phase during which humans normally dream. Brain waves during REM sleep are similar to brain waves when awake, but most muscles in the sleeper are paralyzed, so the sleeper cannot act out their dreams. A grown up sleeps for around two hours in REM sleep.
Studies on sleep deprivation suggest that REM sleep helps learning new or complex things, as people who were deprived of REM sleep showed that the ability was lessened. So far, however, there have only been hypotheses on why REM sleep is important.
Neuroscientists at the Max Planck Institute for Brain Research in Frankfurt, Germany were conducting a study on Australian dragons when they suddenly found information that led them into a different direction. Initially, the scientists were trying to find out what kind of visual information the dragons use to chase and find treats. In order to do that they measured their brain activity over the course of several weeks. Naturally, the electrodes were left in place over night.
The team was surprised that the recording of the sleeping dragons’ brain waves showed that a cycle of two different patterns occurred. While other scientists had specifically conducted studies on reptiles to find information about their sleep patterns, these scientists simply stumbled over the information by chance.
Once they began to suspect that they were likely seeing REM sleep in the dragons they began to dig deeper. They installed infrared cameras that were able to film the sleeping dragons’ eyes and indeed found that their eyelids moved just as rapidly as in other animals during REM sleep. The dragons sleep for between six and ten hours a day and showed the alternating pattern throughout their entire sleep phase.
In the reptiles, the two observed frequencies changed at a much higher rate than they do in humans – forty seconds for the lizards and around ninety minutes in humans. But the fact that this kind of alternating pattern exists in the first place strongly suggests that reptiles, like mammals and birds, go through REM and NREM phases.
It is possible that the alternating sleep patterns are actually something that mammals, birds and reptiles inherited from their common ancestor. It is also possible that reptiles, like humans, use the different sleep phases to form new memories.
Sleep between humans and other animals still varied greatly. Generally, carnivores sleep much more than herbivores, as herbivores need the longer phases of being awake to gather the nutrition they need. Many animals can also sleep while standing up but have to lie down for short bursts of REM sleep, much shorter than in humans. Many aquatic mammals, as well as some birds, sleep with only one part of the brain, possibly as a protection against predators, drowning or crashing.
Despite a similar REM and NREM pattern, it is unlikely that animals can actually dream during the REM phases. Further research will be needed in order to determine, how similar the human REM sleep is to that of the REM sleep of other animals. In the dragons’ case, for example, the measured brain waves originated from the dorsal ventricular ridge, rather than from the hippocampus, where mammals’ slow-wave sleep brain waves originate from.