Rem Sleep: A Universal Mammalian Trait?

Sleep is a biological requirement for almost all animals, except for the most basal species with no brain or only a rudimentary brain. It has been observed in mammals, birds, reptiles, amphibians, fish, and even insects. The internal circadian clock promotes sleep at night for diurnal organisms (such as humans) and during the day for nocturnal organisms (such as rats).

Sleep can be defined physiologically or behaviourally. Physiological sleep is characterised by reversible unconsciousness, 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. The physiological definition is more applicable to birds and mammals, while the behavioural definition is more often used for other animals.

REM (rapid-eye movement) sleep is a distinct phase of sleep during which the brain displays bursts of electrical activity similar to those of a waking brain. It is associated with dreaming in humans, and has been observed in many mammals, birds, and some reptiles, invertebrates, and possibly even spiders. However, some marine mammals, such as whales and dolphins, do not appear to experience REM sleep.

REM sleep is not essential for dreaming, as humans also dream during non-REM sleep. The function of REM sleep is not yet fully understood, but it has been linked to memory consolidation, brain development, and the maintenance of the circuitry needed for waking activities.

REM sleep in birds

Sleep in birds shares some similarities with that of mammals, including rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Birds can also experience unihemispheric slow-wave sleep, where one half of the brain sleeps while the other remains awake. This type of sleep has also been observed in dolphins and whales. Unihemispheric sleep is thought to have evolved in aquatic mammals because they must return to the surface of the water to breathe, and in birds to avoid predation, demonstrating homoplasy in the two groups.

Birds can quickly transition between states of wakefulness, non-REM sleep and REM sleep. When awake, the electroencephalogram (EEG) is activated, muscle tone is typically high and variable, and the bird is often moving. 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 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 restore their arousal thresholds during sleep. During their short eye-open periods, sleeping birds can mobilise almost instantaneously when threatened by a predator. Birds have been found to rely on flock size and height for predatory precautions.

The amount of sleep necessary to function can vary by species. Pectoral sandpipers migrate from the Southern Hemisphere to the Arctic Circle, their mating ground. Since the sandpipers are polygamous, they mate (or search for a mate) for the duration of daylight. Males do not require as much sleep during this time; some have been observed to give up 95% of their sleep time during the 19 mating days.

Light can also affect avian sleep. Exposure to natural and artificial sources of light can regulate and suppress sleep in diurnal birds. Sleep is homeostatically regulated in birds, meaning that sleep which is lost can be recovered by sleeping more; lost non-REM sleep can also be recovered by sleeping more intensely, as indicated by increased slow-wave activity in the EEG.

Sleep in birds has been found to benefit almost all aspects of animal biology, including the maintenance of the brain and central nervous system. Sleep is generally thought to be important for optimal cognitive functioning. Multiple cognitive processes in birds depend on sleep, including imprinting, song learning and auditory discrimination learning.

REM sleep in monotremes

Monotremes are considered to be one of the evolutionarily oldest groups of mammals, and as such, they have been of particular interest in the study of mammalian sleep. Early studies of monotremes concluded that they did not experience REM sleep, and this was used to support the hypothesis that REM sleep evolved after the divergence of monotremes from the rest of the mammalian evolutionary line.

However, more recent research has challenged this assumption. Neuronal recording from mesopontine regions in the echidna, for example, has shown that despite the presence of a high-voltage cortical electroencephalogram (EEG), brainstem units fire in irregular bursts that are intermediate in intensity between the regular non-REM sleep pattern and the highly irregular REM sleep pattern seen in placentals. This suggests that the echidna displays brainstem activation during sleep with high-voltage cortical EEG.

Further research on the platypus revealed sleep with vigorous rapid eye, bill, and head twitching, identical to the behaviour that defines REM sleep in placental mammals. The platypus not only has REM sleep but has more of it than any other animal. The lack of EEG voltage reduction during REM sleep in the platypus has been compared to the sleep seen in neonatal sleep in placentals. The very high amounts of REM sleep in the platypus also fit with the increased REM sleep duration seen in altricial mammals.

These findings suggest that REM sleep may have originated earlier in mammalian evolution than previously thought and support the hypothesis that REM sleep, or a precursor state with aspects of REM sleep, may have had its origin in reptilian species.

REM sleep in cetaceans

Sleep is a state of reduced responsiveness and reduced activity, which is homeostatically regulated. It is defined by a set of behavioural and physiological criteria. Sleep is a vital function for all animals, except for basal species with no brain or only a rudimentary brain. Sleep has been observed in mammals, birds, reptiles, amphibians, fish, and, in some form, in insects. The amount of sleep required varies across species, with some species foregoing sleep for extended periods and some engaging in unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake.

REM sleep is a sleep stage characterised by rapid eye movement, muscle atonia, and vivid dreams. It is present in all terrestrial mammals so far examined, occupying the greatest amount of the 24-hour cycle in the egg-laying mammal platypus. Convincing evidence for this state in reptiles, fish and insects is lacking.

Some marine mammal species do not show evidence for REM sleep, and convincing evidence for this state in reptiles, fish and insects is lacking. The enormous variation in the nature of rest and sleep states across the animal kingdom and within the mammalian class has important implications for understanding the evolution and functions of sleep.

REM sleep in pinnipeds

Pinnipeds, or semi-aquatic mammals, can sleep both on land and in water. They include three families: the Otariidae (eared seals), the Phocidae (true seals), and the Odobenidae (walruses).

Sleep in Otariids or Eared Seals

Otariids include fur seals and sea lions, which can sleep on land and in water. They display both bilateral slow-wave sleep (BSWS) and asymmetrical slow-wave sleep (ASWS) or unihemispheric slow-wave sleep (USWS). ASWS/USWS is similar to the sleep observed in cetaceans, with one cerebral hemisphere exhibiting low-voltage activity resembling the waking state, while the other shows high-voltage slow waves.

During USWS, the eye contralateral to the waking hemisphere is predominantly open or in an intermediate state, while the eye on the sleeping side is usually closed or in an intermediate state. The opening of both eyes is highly correlated with waking, while the asymmetrical eye state is correlated with sleep.

When sleeping in water, fur seals and sea lions adopt a lateral posture, with one foreflipper paddling to stabilize their position, and the other three flippers above the water. This posture prevents heat loss and facilitates breathing. They also sleep in a prone position, with minimal movement, and no noticeable correlation between EEG asymmetry and motion.

REM sleep in water was examined in fur seals kept without access to land for several weeks. REM sleep was suppressed or substantially reduced, with no apparent rebound when the seals returned to land.

Sleep in Phocids or True Seals

Phocids include species such as the gray seal, Caspian seal, harp seal, northern elephant seal, and harbor seal. They usually hold their heads withdrawn into their bodies during slow-wave sleep and rest their heads on the ground at the onset of REM sleep.

True seals sleep in water by floating motionless at the surface, at depth, or on the pool floor. They wake up to breathe when sleeping at the surface but do not always awaken when sleeping at a depth. They can sleep during long apneas, which may last several minutes, and their breathing becomes regular during REM sleep.

Sleep in Odobenids or Walruses

Walruses are the only living species in the Odobenidae family. They sleep on land in a recumbent position and in water by submerging and surfacing for breathing. They always wake up several seconds before surfacing. Walruses can reduce or eliminate sleep for a few days, possibly as an adaptation to the aquatic environment.

REM sleep in humans

Sleep is a complex and mysterious body process that is essential for overall health. Humans, like most animals, have a natural circadian rhythm or internal biological 24-hour clock that regulates sleep and wakefulness.

Humans typically cycle through four stages of sleep: three stages of non-rapid eye movement (NREM) sleep, followed by one stage of rapid eye movement (REM) sleep. The first stage of NREM sleep is the lightest, with slow brain waves and some muscle tone. In the second stage, brain waves slow further and the body temperature and heart rate decrease. The third stage is deep sleep, where the brain produces its slowest brain waves of the night, and the body takes advantage of this to repair injuries and boost the immune system. After the first cycle of NREM sleep, the body enters REM sleep, which is characterised by rapid eye movement, increased brain activity, irregular breathing, and a temporary loss of muscle tone.

REM sleep is important for several reasons. Firstly, it is during this stage that most dreams occur, and it is thought that dreams may be involved in emotional processing. Secondly, REM sleep plays a role in memory consolidation, with the brain processing and committing new information to memory. Thirdly, REM sleep promotes healthy brain development, especially in infants and children. Finally, REM sleep may help us prepare for wakefulness, which could explain why we spend more time in this stage as the night progresses and why we are easier to wake during this stage.

Most adults need about two hours of REM sleep per night, but this can vary depending on daily and biological needs. Missing out on REM sleep can cause difficulty concentrating during the day, excessive sleepiness, and forgetfulness. Over time, chronic REM sleep deprivation is linked to health conditions such as diabetes, depression, obesity, and cardiovascular disease.

While REM sleep has been observed in many land-based species, including humans and other mammals, reptiles, and birds, it can look different depending on the species. For example, owls do not experience rapid eye movements during REM sleep since they cannot move their eyes in their skulls, and some birds only lose muscle tone in certain areas, such as the neck, so that they can remain standing on one foot.

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