Herbie Murray
Sleep is a biological requirement for almost all animals. However, some animals do not experience REM sleep, which is when most dreaming occurs in humans. While REM sleep was once thought to be exclusive to mammals and birds, recent studies have found REM-like activity in cuttlefish, spiders, lizards, and zebrafish. Whales and dolphins may not experience REM sleep at all. The presence of REM sleep in other animals suggests that dreaming may be more widespread in the animal kingdom than previously thought.
| Characteristics | Values |
|---|---|
| Animals that do not have REM sleep | Insects, some marine mammals, some fish, possibly whales |
| Why some animals do not have REM sleep | REM sleep may be too dangerous for some animals, e.g. marine mammals that need to surface to breathe |
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What You'll Learn
Reptiles, insects, and fish do not experience REM sleep
While sleep is common to all living beings, some animals do not experience REM sleep. Reptiles, insects, and fish do not go through REM sleep, a stage in the sleep cycle characterised by rapid eye movement, temporary paralysis of skeletal muscles, and increased brain activity, breathing, and heart rate.
Reptiles
Reptiles have periods of quiescence similar to mammalian sleep, and a decrease in electrical activity in the brain has been observed during these periods. However, the brain activity patterns during reptilian sleep differ from those seen in mammals and other animals. Reptiles may sleep around 16-17 hours a day, and some may enter a state of brumation during cold weather. Some reptiles, like snakes and lizards, do not have eyelids to close when they sleep. Instead, they have a protective scale called a brille that slides across their eyes, giving the appearance of sleeping with their eyes open.
Insects
Insects, such as bees, cockroaches, and scorpions, have quiescent behaviours with elevated arousal thresholds. Circadian changes in sensory response thresholds have been documented in bees. Drosophila (fruit flies) appear to show a behavioural state that satisfies all the criteria of sleep. However, insects do not seem to exhibit REM sleep.
Fish
Fish exhibit periods of inactivity but show no significant reactions to sleep deprivation. Zebrafish, tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night or during the day, depending on their natural activity patterns. Some fish may be able to go without sleep during periods of high activity, such as spawning season or migration.
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REM sleep is associated with memory consolidation
While REM sleep is associated with memory consolidation, the evidence for this hypothesis is weak and contradictory. Animal studies that correlate changes in REM sleep parameters with learning have produced inconsistent results and are confounded by stress effects.
REM sleep is characterised by relaxed muscles, quick eye movement, irregular breathing, elevated heart rate, and increased brain activity. It is one of four stages of sleep, and is associated with dreaming and memory consolidation. It was first discovered in the 1950s when scientists studying sleeping infants noticed distinct periods of rapid eye movement.
REM sleep plays a role in dreaming, memory, emotional processing, and healthy brain development. It is also thought to promote brain development, as newborns spend most of their sleep time in REM. Animals born with less developed brains, such as humans and puppies, spend more time in REM sleep during infancy than those born with more developed brains, like horses and birds.
REM sleep is also linked to the processing of emotions, as dreams may be involved in emotional processing, and the amygdala (the part of the brain that processes emotions) activates during REM sleep.
REM sleep is also associated with memory consolidation, as the brain processes new learnings and motor skills from the day, committing some to memory, maintaining others, and deciding which ones to delete. However, memory consolidation also takes place during deep sleep, a non-REM sleep stage.
Both animal and human studies have documented increased REM sleep after learning. In a study of rats, those who learned a new maze spent more time in REM sleep for nearly a week afterward. Another study monitored the impacts of sleep on the working memory of healthy college students. The group of students that napped in between tests had higher accuracy, and the more time they spent in REM sleep during their nap, the higher their accuracy.
However, studies of the few rare individuals who do not experience REM sleep show that they do not experience problems with memory or learning. That said, REM sleep deprivation disrupts the brain's ability to generate new cells, and multiple studies of both humans and animals suggest that being deprived of REM sleep interferes with memory formation.
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REM sleep may be essential to learning and survival
Sleep is an essential part of our daily routine, and quality sleep is as crucial to our survival as food and water. While the biological purpose of sleep remains a mystery, it is known to affect almost every type of tissue and system in the body, from the brain and heart to metabolism and immune function.
REM sleep, in particular, has been linked to learning and memory consolidation. During REM sleep, the brain transfers information from the motor cortex to the temporal lobe, converting short-term memories into long-term ones. This process, known as sleep spindles, involves one- to two-second bursts of brain waves that rapidly wax and wane at strong frequencies.
Research has shown that sleep, especially REM sleep, improves our ability to solve complex problems creatively. In one study, participants were able to solve 15 to 35% more anagram puzzles when awakened during REM sleep compared to non-REM sleep or while awake during the day. This suggests that REM sleep enhances our ability to solve problems requiring creative thinking.
Furthermore, sleep has been found to play a crucial role in forming long-term memories. MRI scans indicate that the slow brain waves of deep non-REM sleep serve as a courier service, transporting memories from the hippocampus to more permanent storage sites in the brain.
Additionally, sleep deprivation has been linked to various health problems, including high blood pressure, cardiovascular disease, diabetes, depression, and obesity. It is also associated with a decreased ability to concentrate and respond quickly.
While the function of REM sleep is not yet fully understood, its presence in a wide range of animal species suggests that it serves an important purpose in the animal kingdom. REM sleep has been observed not only in mammals but also in birds, reptiles, and some evidence of REM-like activity in spiders, lizards, cuttlefish, and zebrafish.
In conclusion, REM sleep likely plays a vital role in learning and survival by facilitating memory consolidation, enhancing creative problem-solving abilities, and promoting overall health and well-being. Further research is needed to fully understand the role of REM sleep in different species, but it is clear that adequate sleep, including REM sleep, is essential for optimal functioning.
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REM sleep may serve a similar function in emotionally developed animals
REM sleep is a unique brain state that is characterised by muscle paralysis, rapid eye movements, an activated cerebral cortex, and so on. It is thought to be involved in the consolidation of emotional memories.
REM sleep is present in all terrestrial mammals and birds so far examined, occupying the greatest amount of the 24-hour cycle in the egg-laying mammal platypus. However, REM sleep has not been observed in whales, and convincing evidence for this state in reptiles, fish and insects is lacking.
REM sleep is also observed in cuttlefish, which is one of the cephalopods that has the largest and most complex brain. Cuttlefish spend about one-third of the day sleeping, 7% of which is a REM-sleep-like state, characterised by general immobility with occasional twitching, REM, and rapid skin pattern changes including specific darkening around the eyes.
REM sleep in humans and various other mammalian species can be separated by EEG/EMG scoring. In humans, subjects frequently experience emotional and story-like dreams during REM sleep. In rats, sleep deprivation causes weight loss and reduced body temperature.
REM sleep is characterised by a pattern of discharge that closely resembles that of waking in most brain regions. Brainstem neurons are highly active at rates often equal to or exceeding rates in active waking. Cortical neurons also show a waking pattern of activity, with the electroencephalogram (EEG) in many species being indistinguishable from that of waking.
REM sleep is associated with the replay of experiences in some animals. For instance, when researchers looked at the brain electrical activity of sleeping mice that had earlier run a maze, they saw the firing of neurons that help with navigation and are linked with the head's direction, even though the heads of the mice weren't moving. They also saw activity in neurons associated with eye movement. The combination suggests that the mice may have had a dreamlike experience in which they were scanning the environment.
REM sleep is thought to be involved in the consolidation of emotional memories. Theta activity—which describes low-frequency oscillations in the local field potential within the hippocampus, amygdala and neocortex—is a prominent feature of both wake and REM sleep in humans and rodents. Theta coherence between the hippocampus and amygdala drives large-scale pontine-geniculo-occipital (PGO) waves, the density of which predicts increases in plasticity-related gene expression. This could potentially facilitate the processing of emotional memory traces within the hippocampus during REM sleep.
REM sleep may also be involved in the gradual disengagement of successfully consolidated memory traces from the hippocampus—thus mediating the decontextualization of novel memories, allowing generalisation, abstraction, etc.
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REM sleep is associated with processing and storing memories
Sleep is a biological requirement for almost all animals. However, some marine mammals, such as whales and dolphins, may not experience REM sleep. While REM sleep was initially believed to be exclusive to mammals, recent research has identified REM-like activity in spiders, lizards, cuttlefish, and zebrafish.
Research has shown that sleep helps with learning and memory in two ways. Firstly, sleep deprivation impairs attention and focus, hindering the ability to learn efficiently. Secondly, sleep itself plays a role in memory consolidation, which is essential for learning new information. The specific characteristics of brain waves during different sleep stages are believed to be associated with the formation of particular types of memories. For example, REM sleep seems to be involved in declarative memory processes when the information is complex and emotionally charged. On the other hand, slow-wave sleep (SWS) or deep sleep is thought to play a significant role in declarative memory by consolidating newly acquired information.
Additionally, REM sleep is crucial for the consolidation of procedural memory, which involves remembering "how" to perform certain tasks. Studies have shown that REM sleep is necessary for animals to consolidate the memory of performing a task in a maze. Furthermore, motor learning seems to be dependent on the amount of lighter stages of sleep, while certain types of visual learning rely on sufficient deep sleep and REM sleep.
Overall, the current understanding of the relationship between REM sleep and memory suggests that adequate sleep is vital for optimal memory function and learning.
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Frequently asked questions
Insects and fish do not experience REM sleep. However, recent studies have shown that some birds and all mammals, including reptiles, experience REM sleep.
REM stands for rapid eye movement. During this stage of sleep, a sleeping animal's eyes dart about unpredictably, and the body experiences temporary paralysis of skeletal muscles, periodic twitches, and increased brain activity, breathing, and heart rate.
The purpose of REM sleep is still unknown, but it is believed to be linked to memory consolidation and the processing of emotions.
Non-REM sleep is characterised by slow-wave sleep (SWS) and is associated with quiet sleep. On the other hand, REM sleep involves rapid eye movements and resembles wakefulness in terms of brain activity.
