Sleep is a biological requirement for almost all animals, except for the most basal species with no brain or only 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, or rapid eye movement sleep, is a state of sleep characterised by muscle atonia, rapid eye movement, and vivid dreams. It is thought to be involved in memory consolidation and brain maturation.
REM sleep has been identified in birds, but very few avian species have been investigated. The number of studies of amphibian and reptilian sleep is minuscule, with few such studies using rigorous electrophysiological and behavioural indices.
The monotremes – the platypus and the echidna – are the only mammals that hatch from eggs. The echidna has been found to have aspects of REM sleep, but no periods of sleep with the low-voltage EEG and elevated arousal thresholds typical of REM sleep. The platypus, on the other hand, has been found to spend over 8 hours a day in a state with the eye, electromyogram, electrocardiogram, and arousal threshold changes typical of REM sleep.
Therefore, the platypus demonstrates REM sleep.
Characteristics | Values |
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--- | --- |
Animal | Dolphin, Platypus, Echidna, Bearded Dragon, Argentine Tegu, Caiman Slerops, Iguana, Green Iguana, Great White Shark, Hammerhead Shark, Rat, Human, Cat, Dog, Bird, Fish, Insects, Bees, Jellyfish, Nematode, Cuttlefish, Octopus, Chicken, Elephant, Giraffe, Bat, Rabbit, Chimpanzee, Red Fox, Mouse, Lion, Platypus, Chipmunk, Tiger, Armadillo, Leopard, Killer Whale, Whale, Fur Seal, Horse, Cow, Sheep, Dolphin, Whale, Shark |
Sleep Type | REM sleep |
What You'll Learn
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).
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 unihemispheric sleep, a specialised adaptation that enables animals to partially sleep under ecological circumstances when being fully asleep would be disadvantageous. Unihemispheric sleep is not restricted to birds; it has also been described in marine mammals and non-avian reptiles. Unihemispheric 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 vary 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.
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REM sleep in monotremes
Monotremes are egg-laying mammals that are considered to represent one of the evolutionarily oldest groups of mammals. As such, they have been of special interest in the study of mammalian sleep.
Early studies of the echidna led to the conclusion that this monotreme did not experience REM sleep. As monotremes had diverged from the placental and marsupial lines very early in mammalian evolution, this finding was used to support the hypothesis that REM sleep evolved after the start of the mammalian line.
However, more recent research has shown that the echidna displays brainstem activation during sleep with a high-voltage cortical electroencephalogram (EEG). This has encouraged scientists to study sleep in the platypus, which has shown 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 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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REM sleep in reptiles
Research has shown that reptiles, specifically the Australian dragon lizard, experience REM sleep. This discovery was made by scientists at the Max Planck Institute for Brain Research in Germany, who placed probes inside the brains of five Australian dragon lizards to measure electrophysiological activity during sleep.
The Discovery of REM Sleep in Reptiles
Until recently, REM sleep had only been observed in mammals and birds, and it was believed that this sleep stage evolved independently in these two groups. However, the discovery of REM sleep in reptiles suggests that this sleep trait may have emerged much earlier in the evolutionary ancestors of mammals, birds, and reptiles.
The Australian Dragon Lizard
The Australian dragon lizard, or Pogona vitticeps, is a type of bearded dragon lizard native to Australia. It is distantly related to birds in the tree of life, making it a good subject for understanding the evolution of sleep.
The Study
In the study, electrodes were placed on the surface of the lizards' brains to record brain activity during sleep. The results showed that the lizards experienced similar sleep stages as humans, including slow-wave sleep, sharp waves, ripples, and REM sleep.
The Implications
The discovery of REM sleep in reptiles has several implications for our understanding of sleep evolution and the function of sleep. Firstly, it suggests that sleep, including REM sleep, may have evolved from a common ancestor of reptiles, birds, and mammals. Secondly, it provides insight into the purpose of REM sleep, which is still not fully understood.
The Function of REM Sleep
REM sleep is believed to be important for memory consolidation, emotional regulation, and brain development. The high amount of REM sleep in newborns may be linked to the development of the brain and sensory-motor systems. As animals mature, the amount of REM sleep decreases, suggesting that REM sleep plays a more significant role in early life.
Differences in Sleep Cycles
It is important to note that while lizards experience similar sleep stages as humans, the length of their sleep cycles is much shorter. A fast sleep cycle for a lizard is around 80 seconds, compared to 30 minutes for a house cat and 60-90 minutes for humans.
In conclusion, the discovery of REM sleep in reptiles, specifically the Australian dragon lizard, has provided valuable insights into the evolution and function of sleep. Further research in this area may help us better understand the purpose of sleep and its role in different animal species.
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REM sleep in insects
Insects do seem to sleep, but the kind of sleep they have is different from the kind of sleep that humans experience. Insects do not seem to go into homeostatic sleep like humans do, and they do not seem to experience periods of REM sleep. However, insects 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. Insects have been observed to have prolonged periods of rest, which scientists have called torpor—the closest thing to genuine sleep that insects undergo. During torpor, insects exhibit decreased physiological activity, such as a lower body temperature and a lower metabolic rate.
The most studied insect in this area is the common fruit fly. Researchers from UW-Madison explored possible sleep patterns in fruit flies and noticed that they 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. Fruit flies that were interrupted during their rest periods had a much tougher time learning certain tasks and behaviours and forgot conditioning quicker than the fruit flies that were allowed to rest.
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.
Scientists have established that these sleep-like states are important for insect cognitive function. Insects do seem to sleep, but the kind of sleep they have is a bit different than the kind of sleep that humans experience. Still, studying sleep in insects can give us important clues about the role and evolution of sleep throughout the history of life on Earth.
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REM sleep in marine mammals
Marine mammals, such as dolphins, whales, and seals, have unique sleep patterns. They engage in unihemispheric sleep, allowing them to swim, surface for breathing, and maintain vigilance while resting. This is a survival adaptation to the challenges posed by the aquatic environment, such as the need to surface for breathing and thermoregulation.
During unihemispheric sleep, one brain hemisphere displays slow waves characteristic of non-rapid eye movement (NREM) sleep, while the other hemisphere remains awake or exhibits low-voltage activity similar to the waking state. This enables them to simultaneously rest and engage in essential activities like swimming and breathing.
Unlike terrestrial mammals, marine mammals typically do not exhibit rapid eye movement (REM) sleep, which is characterised by rapid eye movements, muscle atonia, and increased heart and breathing rate. The absence of REM sleep in marine mammals is likely an adaptation to their environment, as prolonged periods of immobility and reduced responsiveness during REM sleep could increase their vulnerability to predators and the risk of drowning.
However, some marine mammals, such as seals, do exhibit REM sleep when sleeping on land. Additionally, muscle jerks and body twitches, which are features of REM sleep, have been observed in resting cetaceans.
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Frequently asked questions
REM sleep has been observed in a wide range of animals, including mammals, birds, reptiles, and fish.
Sleep appears to be a biological requirement for all animals except for basal species with no brain or only a rudimentary brain.
If sleep were not essential, one would expect to find animal species that do not sleep at all, do not need recovery sleep after staying awake longer than usual, or do not suffer any serious consequences as a result of a lack of sleep. These symptoms are not seen in complex animals, and sleep is thus considered necessary to them.
Sleep patterns vary widely among species, with some foregoing sleep for extended periods and some engaging in unihemispheric sleep, in which one brain hemisphere sleeps while the other remains awake.