Rem Sleep: A Universal Language Of Rest?

Sleep is a biological requirement for nearly all animals, except for those with no brain or only a very simple one. 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 is a distinct sleep state characterised by rapid eye movement, muscle atonia, brain activation, and dreaming. It is found in many mammals and birds, and some reptiles, birds, and aquatic invertebrates experience REM sleep or its ancestral form. However, REM sleep does not occur in all species, and its presence or absence is not clearly linked to phylogeny.

REM sleep is thought to be important for brain maturation, memory consolidation, and the regulation of noradrenergic receptor sensitivity. However, the exact function of REM sleep and the reason for its presence in some species and absence in others remain unclear.

REM sleep in birds and reptiles

Similarities

Birds and reptiles, like mammals, can experience two distinct sleep states: non-rapid eye movement (NREM) sleep and REM sleep.

During REM sleep in birds and reptiles, there is a period of wake-like brain activity, similar to mammals. This is accompanied by rapid eye movements, and muscle twitches.

Differences

Unlike mammals, birds and reptiles do not always show muscle atonia during REM sleep. For example, birds can maintain some muscle tone during REM sleep to control their head posture.

REM sleep in fish

While it was previously believed that REM sleep was exclusive to mammals, birds, and reptiles, recent research has shown that fish also experience REM sleep.

Zebrafish

Zebrafish (Danio rerio) are transparent during their first few weeks of life, which has allowed researchers to peer directly into their organs without the need for surgery. In 2019, a team of scientists led by Philippe Mourrain, a neurobiologist at Stanford University, conducted a study on the sleep patterns of zebrafish. They found that zebrafish experience two stages of sleep that are similar to slow-wave sleep and REM sleep in humans.

During the first stage, the zebrafish exhibited brain activity that resembled slow-wave or deep sleep in humans, which is characterised by low brain activity and is thought to give the body an opportunity to recuperate and may also play a role in memory consolidation.

In the second stage, the zebrafish displayed brain activity similar to REM sleep in humans, which is characterised by rapid eye movements and high brain activity. However, unlike humans, zebrafish do not appear to move their eyes during this stage.

Other Fish Species

The sleep habits of different fish species vary greatly. Some fish can receive oxygen while remaining nearly stationary, while others, like some sharks and rays, need to make larger movements to get enough water flowing over their gills. Due to unihemispheric sleep, these larger fish can keep swimming while they sleep, with one half of their brain remaining active while the other half sleeps.

Some fish enter a state of estivation, which is similar to hibernation but occurs in dry rather than cold climates. Fish may estivate during periods of food or water deprivation, and their metabolism slows down drastically during this state, allowing them to survive for extended periods.

Similarities and Differences

While the sleep patterns of zebrafish share similarities with those of humans and other vertebrates, there are also some differences. For example, the equivalent of slow-wave sleep in zebrafish was much slower than in humans. Additionally, the eyes of zebrafish remain still during their version of REM sleep, while the eyes of humans typically move rapidly.

Further research is needed to determine whether multistage sleep is universal among vertebrates and to understand the functional and evolutionary significance of REM sleep.

REM sleep in insects

Insects do not seem to experience periods of REM sleep. However, recent studies have shown that spiders may have REM-like sleep and may even dream. Jumping spiders have REM-like twitches when they sleep, suggesting that dreams may be more widespread in the animal kingdom than previously thought.

REM sleep, or rapid eye movement sleep, is a behavioural state, a brain state, a dream state, and a paradoxical state. It is produced by complex and anatomically distributed neural circuits that give rise to a variety of individual components, including muscle paralysis, rapid eye movements, and an activated cerebral cortex.

REM sleep is present and tightly regulated in every vertebrate species in which it has been carefully investigated. Sleep is also present in invertebrates, and an extensive analysis in Drosophila melanogaster has shown that sleep in fruit flies shares most of the fundamental features that characterise sleep in mammals.

However, it is difficult to study insect brain activity in the same way as larger animals, and so it is unclear whether insects experience REM sleep.

Unihemispheric sleep

The amount of time spent sleeping during the unihemispheric slow-wave stage is considerably less than the bilateral slow-wave sleep. In the past, aquatic animals, such as dolphins and seals, had to regularly surface in order to breathe and regulate body temperature. Unihemispheric sleep might have been generated by the need to perform these vital activities simultaneously with sleep.

On land, birds can switch between sleeping with both hemispheres to one hemisphere. Due to their poorly webbed feet and long wings, which are not completely waterproof, it is not energetically efficient for them to make rest stops or land on water, only to take flight again. Using unihemispheric slow-wave sleep, birds are able to maintain environmental awareness and aerodynamic control of wings while obtaining the necessary sleep they need to sustain attention during wakefulness. Their sleep is more asymmetric in flight than on land, and they sleep mostly while circling air currents during flight. The eye connected to the awake hemisphere of their brain is the one facing the direction of flight. Once they land, they pay off their sleep debt, as their REM sleep duration significantly decreases and slow-wave sleep increases.

Despite the reduced sleep quantity, species having unihemispheric sleep do not present limits at a behavioural or healthy level. Cetaceans, such as dolphins, show preserved health as well as great memory skills. Indeed, cetaceans, seals, and birds compensate for the lack of complete sleep with efficient immune systems, preserved brain plasticity, thermoregulation, and restoration of brain metabolism.

Sleep in monotremes

Monotremes are mammals of the order Monotremata and are the only group of living mammals that lay eggs. The extant monotreme species are the platypus and the four species of echidnas. They have many characteristics of the reptilian predecessors of the mammals.

The echidna was originally thought to experience no rapid eye movement (REM) sleep. However, a more recent study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an environmental temperature of 25°C (77°F). The study also observed very little REM at reduced temperatures of 15°C (59°F) and 20°C (68°F), and a substantial reduction at the elevated temperature of 28°C (82°F).

The platypus, on the other hand, was found to have more REM sleep than any other mammal. The platypus was seen to have sleep with vigorous rapid eye, bill, and head twitching, identical in behaviour to that which defines REM sleep in placental mammals. The platypus not only has REM sleep, but it had more of it than any other animal.

The echidna combines REM and non-REM aspects in a single sleep state. By using neuronal recording from mesopontine regions in the echidna, researchers found that despite the presence of a high-voltage cortical electroencephalogram (EEG), brainstem units fire in irregular bursts intermediate in intensity between the regular non-REM sleep pattern and the highly irregular REM sleep pattern seen in placentals. Thus, the echidna displays brainstem activation during sleep with high-voltage cortical EEG.

The lack of EEG voltage reduction during REM sleep in the platypus, and during the REM sleep-like state of the echidna, has some similarity to the sleep seen in neonatal sleep in placentals. The very high amounts of REM sleep seen in the platypus also fit with the increased REM sleep duration seen in altricial mammals. These findings suggest that REM sleep originated earlier in mammalian evolution than had previously been thought and is consistent with the hypothesis that REM sleep, or a precursor state with aspects of REM sleep, may have had its origin in reptilian species.

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