Animals That Experience Rem Sleep: A Comprehensive Overview

Sleep is a biological requirement for almost all animals, except for basal species with no brain or a rudimentary brain. Sleep patterns vary widely among species, with some foregoing sleep for extended periods and some engaging in unihemispheric sleep, where one brain hemisphere sleeps while the other remains awake.

REM sleep has been observed in birds, but very few avian species have been investigated. REM sleep has also been observed in mammals, and recent studies suggest that REM sleep, or ancestral forms of REM sleep might be found in non-mammalian or -avian species such as reptiles.

The existence of two distinct sleep states only in endotherms has been thought to be the other clue to understanding the evolution of sleep. However, recent observations in reptiles indicate more convincingly than earlier studies that they could possess two electrophysiological sleep states.

The two distinct sleep states in mammals and birds are non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. REM sleep is a distinct state that was first discovered while observing children's eye movements during sleep. During this phase, cortical activity resembles wake activity, challenging the notion of sleep as a uniformly low activity state.

REM sleep is a sleep state in which there is repetitive phasic activation of brainstem reticulo-motor systems. The simultaneous activation of excitatory and inhibitory motor systems makes it necessary to conduct careful monitoring of motor output and perhaps the monitoring of central motor systems before concluding that no motor activation is occurring.

The presence of REM sleep in all three branches of the mammalian tree suggests that it or a very similar state was present in the earliest mammals. REM sleep is present in birds. The ubiquity of REM sleep in mammals and its presence in birds is most parsimoniously explained if one hypothesizes a single origin of this state in the common reptilian ancestors of birds and mammals.

The common cuttlefish (Sepia Officinalis), one of the cephalopods that has the largest and most complex brain, has been shown to possess two different sleep-like states: a quiescent sleep state and a REM-sleep like state.

REM sleep in birds and mammals

REM sleep is a state of sleep characterised by rapid eye movement, muscle atonia, and brain activation. It is observed in both birds and mammals, and is thought to be important for brain development and function. However, the exact function of REM sleep is still not fully understood.

REM sleep in mammals has been linked to endothermy, the ability to produce heat internally. This is because REM sleep is accompanied by a suspension of thermoregulatory mechanisms, and endothermic animals are able to suspend internal heat production during this state.

The exact evolutionary origins of REM sleep are still unclear, but it is thought to have emerged earlier than previously thought. Recent evidence of REM-like sleep in fish suggests that the divergence between birds and mammals may have occurred much earlier.

Overall, REM sleep in birds and mammals shares many similarities, but there are also some key differences that are yet to be fully understood.

REM sleep in reptiles

In 2016, a study found evidence of REM sleep in the Australian dragon lizard, *Pogona vitticeps*, or the bearded dragon. This was the first time reptiles were observed to experience REM sleep, which was previously thought to occur only in mammals and birds.

The Study

The researchers from the Max Planck Institute for Brain Research in Germany placed probes inside the brains of five bearded dragons to measure electrophysiological activity during sleep. They found that the lizards experienced REM sleep and slow-wave sleep, with some telltale signs of these sleep stages found in a more primitive brain region, the dorsal ventricular ridge, instead of the hippocampus, where they are found in mammals.

Sleep Cycles

The lizard's sleep cycle is about 80 seconds long at 27°C, compared to 30 minutes in cats and 60-90 minutes in humans. While humans experience four or five long slow wave/REM sleep cycles nightly, the lizards averaged 350 80-second-long cycles.

Implications for Sleep Evolution

The discovery that reptiles share these sleep stages with mammals and birds suggests that these sleep traits may have emerged earlier than previously thought, in the common evolutionary ancestors of the three groups. This common ancestor is believed to be small lizard-like animals that lived between 300 and 320 million years ago.

Similarities and Differences

Like mammals and birds, reptiles have quiescent periods similar to mammalian sleep, and a decrease in electrical activity in the brain has been registered when the animals are asleep. However, the EEG pattern in reptilian sleep differs from what is seen in mammals and other animals.

Unihemispheric Sleep

Some reptiles, like birds and aquatic mammals, are capable of unihemispheric sleep, where one cerebral hemisphere sleeps while the other remains awake. This allows them to receive stimuli from their environment and periodically surface to breathe when immersed in water.

The identification of REM sleep in reptiles contributes to our understanding of the evolution of sleep and its role in the animal kingdom. Further studies on different reptilian species and sleep states are needed to fully comprehend the nature and function of sleep across species.

Sleep in non-amniotic vertebrates

Non-amniotic vertebrates, or anamniotes, are vertebrates that develop without extra-embryonic membranes. They are not a formal classification group, as they are associated by the absence of a characteristic. Anamniotes include extant and extinct amphibians, as well as two groups of extinct reptile-like animals: anthracosaurs and batrachosaurs.

Anamniotes are distinguished from amniotes by the lack of three extra-embryonic membranes: the amnion, chorion, and allantois. These membranes are essential for embryonic protection, gas exchange, and metabolic waste disposal or storage. Amniotes, on the other hand, are the evolutionary branch of tetrapods in which the embryo develops within these protective membranes. This arrangement allows amniotes to reproduce on dry land, either by laying shelled eggs or nurturing fertilized eggs within the mother.

While sleep has been observed in amniotes, including mammals, birds, and reptiles, it is unclear whether anamniotes exhibit REM sleep. Sleep in anamniotes, particularly in amphibians and reptiles, has been the subject of limited research.

Reptiles, which are non-amniotic vertebrates, have quiescent periods similar to mammalian sleep, and a decrease in electrical activity in the brain has been observed during these periods. However, the EEG patterns in reptilian sleep differ from those seen in mammals and other animals. Reptiles require stronger stimuli to awaken after sleep deprivation, suggesting that the sleep which follows deprivation is compensatorily deeper.

In 2016, a study reported the existence of REM- and NREM-like sleep stages in the Australian dragon Pogona vitticeps. This finding suggests that reptiles may indeed experience REM sleep. However, more research is needed to confirm this and understand the nature of sleep in non-amniotic vertebrates.

Sleep in invertebrates

Sleep in crayfish, a type of crustacean, has been studied through both behavioural and electrophysiological methods. Crayfish have been found to display a variety of behaviours, the most common being alert and interactive, motionless, and lying on one side on the surface of the water. When crayfish lie on their side, they do not seem aware of their surroundings and have a high threshold for awakening with sensory stimulation. This suggests that they are sleeping.

The brain electrical activity of crayfish has also been studied. In alert animals, the brain electrical activity is comprised of numerous spikes on an almost flat baseline. However, when a crayfish lies on its side, the number of spikes decreases and the record becomes dominated by slow waves. This change in brain activity is a characteristic of sleep.

The sleep of other invertebrates, such as bees, has also been studied. During the night, some solitary bees clamp their mandibles to a stick and remain in this position until the sun lights up. Experiments have also been conducted on uncommon animals, such as the echidna, platypus, and sloths, and results show that they also seem to follow the general pattern of sleep seen in mammals.

The evolutionary origin of distinct NREM and REM sleep

The evolutionary origin of distinct non-rapid eye movement (NREM) and rapid eye movement (REM) sleep states is a topic of ongoing scientific research. While the functions of these distinct sleep states are poorly understood, recent studies suggest that REM sleep or its ancestral forms might be found in non-mammalian or non-avian species such as reptiles. This suggests that REM sleep and NREM sleep evolved earlier than previously thought.

REM sleep is characterised by features such as wake-like brain activity, muscle atonia, rapid eye movements, and muscle twitches. In mammals, REM sleep is also accompanied by fluctuations in heart rate and respiration, and whisking. In humans, REM sleep is associated with dreams.

The existence of two distinct sleep states only in endotherms has been thought to be a clue to understanding the evolution of sleep. However, more recent studies suggest that a REM sleep-like state could exist in ectotherms, such as reptiles, teleost fishes, or even cuttlefish.

The standard method to define sleep in animals with a well-developed cerebral cortex, like mammals and birds, is the observation of a pattern of coordinated changes in electroencephalogram (EEG) and electromyogram (EMG) readings. In other animals, sleep is usually defined solely by behavioural criteria:

• Stereotypic posture
• Consolidated quiescence at a particular time of day
• Increased arousal threshold
• A compensatory increase in this behaviour after deprivation

In reptiles, the cortical EEG does not allow unambiguous identification of sleep states. However, recent findings have provided new information suggesting that different sleep states could exist in lizards.

In the Australian bearded dragon (*Pogona vitticeps*) and the Argentine tegu (*Salvator merianae*), two sleep-like states have been observed. The first state is rich in delta frequency (0.5-4Hz) and characterised by numerous negative high-amplitude sharp waves. The second state is accompanied by beta waves (10-40Hz) and dominant oscillations at 15Hz frequency, and the lizards tend to show eye movements associated with these brain activities.

In zebrafish, sleep has been defined solely by behavioural criteria based on periods of quiescence associated with a specific posture. A recent study suggested that there are at least two sleep states in zebrafish. The first state is characterised by high spontaneous and desynchronous activity in the dorsal pallium, while the second state is characterised by highly synchronous bursts of activity in the same region.

In the common cuttlefish (*Sepia Officinalis*), two different sleep-like states have been observed: a quiescent sleep state and a REM sleep-like state.

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