The Mystery Of Rem Sleep: Humans And Animals Compared

Sleep is a biological requirement for almost all animals, except for some of the most basal species with no brain or only a rudimentary brain. Sleep is defined physiologically as a state of reversible unconsciousness, with specific brainwave patterns, sporadic eye movement, and loss of muscle tone. Behaviourally, sleep is characterised by minimal movement, non-responsiveness to external stimuli, the adoption of a typical posture, and occupation of a sheltered site.

REM (rapid-eye movement) sleep is a distinct phase of sleep characterised by rapid eye movements, muscle twitching, and brain activity similar to that of a waking state. REM sleep is thought to be important for memory consolidation, brain maturation, and the development of the sensorimotor system.

REM sleep has been observed in many mammals, birds, and some reptiles, invertebrates, and fish. However, it is unclear whether REM sleep is present in all animals, and some species appear to have very little REM sleep or none at all. For example, whales and dolphins do not show typical REM sleep behaviours, and some evidence suggests that REM sleep is absent in reptiles, fish, and insects.

Recent studies have challenged the idea that REM sleep is absent in non-mammalian and non-avian species. For instance, researchers have found REM-like activity in cuttlefish, octopuses, bearded dragons, zebrafish, and spiders. These findings suggest that REM sleep may be far more widespread in the animal kingdom than previously thought and raise intriguing questions about whether non-human animals dream.

REM sleep in birds

Birds, like mammals, have two main types of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. These two states can be distinguished from each other and from wakefulness using brain activity (based on the electroencephalogram, or EEG), muscle tone (electromyogram, EMG) and behaviour (accelerometry and/or video recordings).

REM sleep is generally characterised by a suite of features in addition to rapid eye movements: the temporary paralysis of skeletal muscles, periodic body twitches and increases in brain activity, breathing and heart rate. Observed in sleeping infants in 1953, REM was soon identified in other mammals such as cats, mice, horses, sheep, opossums and armadillos. Birds have REM sleep, which comes with twitching bills and wings, and a loss of tone in the muscles that hold up their heads.

Birds can quickly transition between states of wakefulness, non-rapid eye movement (non-REM) sleep and REM sleep. When birds are awake, the EEG is activated, muscle tone is typically high and variable (shown by the electromyogram or EMG), and the bird is often moving (shown by recordings of accelerometry or video). Non-REM sleep is characterised by slow (4 Hz) large waves in the EEG, typically accompanied by relaxed skeletal musculature and quiescent behaviour; many birds (including pigeons) can also have one or both eyes open. REM sleep is characterised by wake-like patterns in the EEG, often (but not always) relaxed skeletal musculature from the preceding non-REM sleep level, eye movements behind closed eyelids and behavioural restfulness.

Birds can engage in NREM sleep with one hemisphere at a time, or more deeply with one hemisphere than the other (asymmetric sleep) – a phenomenon most pronounced in dolphins. Such asymmetric or unihemispheric NREM sleep occurs with one eye open, enabling birds to visually monitor their environment for predators. Frigatebirds primarily engage in this form of sleep in flight, perhaps to avoid collisions with other birds. In addition to interhemispheric differences in NREM sleep intensity, the intensity of NREM sleep is homeostatically regulated in a local, use-depended manner within each hemisphere. Furthermore, the intensity and temporo-spatial distribution of NREM sleep-related slow waves varies across layers of the avian hyperpallium – a primary visual area – with the slow waves occurring first in, and propagating through and outward from, thalamic input layers. Slow waves also have the greatest amplitude in these layers.

Although most research has focused on NREM sleep, there are also local aspects to avian REM sleep. REM sleep-related reductions in skeletal muscle tone appear largely restricted to muscles involved in maintaining head posture. Other local aspects of sleep manifest as a mixture of features of NREM and REM sleep occurring simultaneously in different parts of the neuroaxis. Like monotreme mammals, ostriches often exhibit brainstem-mediated features of REM sleep (muscle atonia and REMs) while the hyperpallium shows EEG slow waves typical of NREM sleep. Finally, although mice show slow waves in thalamic input layers of primary sensory cortices during REM sleep, this is not the case in the hyperpallium of pigeons, suggesting that this phenomenon is not a universal feature of REM sleep.

REM sleep in reptiles

Reptiles, like humans, experience rapid eye movement (REM) sleep. This was discovered in a 2016 study, where researchers placed probes inside the brains of five lizards called Australian bearded dragons. The findings suggest that reptiles may dream, too.

The discovery that reptiles share these important sleep stages with mammals and birds suggests that the sleep traits emerged earlier than previously thought in the evolutionary ancestors of the three groups. Reptiles are ectotherms and tend to minimise energy expenditure, spending most of their time waiting for prey, basking, or resting. However, their behavioural sleep is distinct from simple rest as they display a stereotypic position in a specific location, for example, curling into a shelter or lying with their head on a leaf.

The existence of two distinct sleep states in reptiles, fish, and even cuttlefish has opened up new avenues of research into understanding REM sleep.

REM sleep in fish

While it is unclear whether fish dream, research has shown that they experience REM sleep.

REM sleep, or Rapid Eye Movement sleep, is characterised by rapid eye movements, muscle twitching, and brain activity similar to that of a waking state. In humans, it is the stage of sleep in which we dream.

Zebrafish, or Danio rerio, have been found to experience two stages of sleep, one of which is similar to the REM sleep experienced by humans. This discovery was made by a team of scientists led by neurobiologist Philippe Mourrain of Stanford University. Using fluorescent markers, the researchers were able to observe the electrical signals of zebrafish during sleep, as their skin is transparent during the first few weeks of life. They found that the brain activity, heart rate, and eye and muscle movement of the zebrafish during one of their sleep stages was similar to that of humans during REM sleep.

REM sleep in zebrafish shares some characteristics with that of humans, but there are also differences. For example, the zebrafish equivalent of slow-wave sleep was much slower than that of humans.

This discovery suggests that multistage sleep may be common among vertebrates and may have arisen over 450 million years ago. Further research is needed to determine whether other types of fish experience REM sleep and whether they dream.

REM sleep in insects

Insects do not seem to experience periods of REM sleep. However, they do seem to alternate between periods of rest and alertness. Scientists have established that insects have some kind of circadian rhythm that determines periods of stasis and alertness.

Scientists have called these prolonged periods of rest torpor, which is the closest thing to genuine sleep that insects undergo. During torpor, insects exhibit decreased physiological activity such as lower body temperature and metabolic rate. Torpor is analogous to hibernation in some mammals.

The most studied insect in this area is the common fruit fly. In a widely cited study, researchers from UW-Madison explored possible sleep patterns in fruit flies. They noticed that fruit flies alternate between sleep-like states lasting about three hours and periods of alertness. During these rest periods, fruit flies are relatively unresponsive to external stimuli. Further, the researchers determined that depriving fruit flies of this rest causes significant cognitive problems.

Scientists have established that these sleep-like states are important for insect cognitive function. Other studies on bees show similar findings. When honey bees are deprived of rest, they become unable to perform their characteristic "waggle-dance" that they use to communicate with others. Honey bees also seem to use these periods of rest to consolidate long-term memories.

Overall, insects do seem to sleep, but the kind of sleep they have is a bit different from the kind of sleep that humans experience.

REM sleep in marine mammals

Marine mammals, such as dolphins, whales, and porpoises, are known to use unihemispheric sleep, which allows them to keep one eye open and stay alert while the other half of their brain sleeps. This is particularly useful for dolphins, which tend to keep the eye facing the rest of the pod open while sleeping, to avoid becoming separated.

Unihemispheric sleep has also been observed in birds, which use it to doze while flying. However, birds and dolphins use this technique for different purposes. In a flock of birds, those on the outside of the group keep the eye facing away from the flock open to watch for predators.

Northern elephant seals have been found to enter REM sleep while diving to depths of around 1,000 feet. During this time, they become paralysed and flip upside down, spinning in a slow circle.

While whales do not appear to experience REM sleep, they do exhibit some muscle jerking that might be representative of it.

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