Understanding Rem Sleep: A Comparison Guide

Sleep is a complex and mysterious process that is essential for human health and well-being. While sleeping, the body cycles between different stages of non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. REM sleep, characterised by rapid eye movement, increased brain activity, irregular breathing, and elevated heart rate, is particularly fascinating due to its association with dreaming and its role in memory consolidation, emotional processing, and brain development. Comparing REM sleep with other stages of sleep, such as NREM sleep, provides insights into the unique characteristics and functions of this crucial sleep stage.

Dreaming

There is a common misconception that dreaming only happens during REM sleep. This is due to early research in the 1950s, which discovered vivid dreaming during REM sleep. However, it is now known that dreaming can occur during the early, non-REM stages of sleep as well.

During REM sleep, the brain activity is similar to that of when we are awake. It is characterised by relaxed muscles, quick eye movement, irregular breathing, an elevated heart rate, and increased brain activity. Dreaming occurs during REM sleep, and the dreams are usually more vivid than those during non-REM sleep. However, REM sleep is not the only stage in which dreams occur.

Dreams have been studied in a variety of ways, with scientists divided as to whether dreams are simply a product of random neurons firing during sleep, or if they are something more. Sigmund Freud's "The Interpretation of Dreams", published in 1900, speaks to the complex relationship between conscious and unconscious thought processes.

Recent studies have shown that REM sleep can affect how accurately people can read emotions and process external stimuli. For example, people who achieved REM sleep during a nap were better able to judge facial expressions afterward.

There are several hypotheses about the purpose of dreams and REM sleep. One theory, known as the REM calibration hypothesis, suggests that norepinephrine, which is associated with stress, builds up during the day and can be reset to normal levels during REM sleep. This may result in the amygdala, the fear centre of the brain, becoming less sensitive to stimuli and less likely to overreact. Another hypothesis is that REM sleep and dreams are involved in the transfer of memories between the hippocampus and neocortex.

Memory consolidation

REM Sleep and Memory Consolidation

REM sleep is characterised by muscle atonia, desynchronised waking-like EEG activity, and a 5-10 Hz hippocampal theta rhythm. REM sleep has been linked to memory consolidation, with studies showing that REM sleep deprivation impairs performance on hippocampal-dependent working memory tasks. However, the evidence supporting this link is weak and contradictory.

Animal studies examining the effects of REM sleep deprivation on learning have produced inconsistent results, and it is difficult to separate the effects of stress from those of REM sleep deprivation. Humans with pharmacologically or lesion-induced REM sleep suppression do not show memory deficits, and other human sleep-learning studies have yielded mixed results. There is no correlation between REM sleep time and learning ability in humans or across species.

Non-REM Sleep and Memory Consolidation

Non-REM sleep, which is divided into four stages, occurs earlier and lasts longer than REM sleep. It is characterised by slow oscillation/delta activity and is important for physical rest. Non-REM sleep has been shown to have a positive effect on tasks such as remembering learned faces and words.

The Interaction of REM and Non-REM Sleep with Memory Consolidation

The active system consolidation hypothesis combines the dual-process and sequential hypotheses, emphasising the importance of the unity of non-REM and REM sleep in memory processes. According to this hypothesis, non-REM sleep, specifically the slow-wave sleep (SWS) phase, strengthens memory traces, while REM sleep stabilises these changes.

Studies have shown that sleep deprivation impairs memory consolidation, and that sleep is necessary for optimal performance in learning tasks. However, the specific role of REM sleep in memory consolidation remains unclear, with some studies suggesting that non-REM sleep may play a more important role.

While sleep is important for memory consolidation, the specific role of REM sleep is still not well understood and requires further research.

Emotional processing

REM sleep is important for the processing of emotional memories, including fear memories. REM sleep has been postulated to facilitate emotional processing of negative stimuli. However, the conditions under which higher amounts of REM sleep lead to decreased or increased emotional responses are unclear.

REM sleep may increase reactivity to emotional stimuli in the short-term and this effect of REM sleep appears to facilitate emotional processing during subsequent nights, leading to reduced intrusive picture memories in the long-term.

REM sleep deprivation is associated with an increased reactivity towards aversive emotional information. Together with the decrease in prefrontal activation, the regulation of emotions gets dysfunctional. Healthy sleep repairs adaptive processing and functional brain activity, integrity of the medial prefrontal cortex- amygdala connections important in emotion regulation processes.

REM sleep may be adaptive to process aversive experiences such as traumatic experiences, by presenting them as strange images and fragmented episodes of related or similar stories.

One of the most important theories for explaining the role of REM sleep in modifying the emotional tone of previous experiences or memories is proposed by Walker and Van der Helm, which argued that REM sleep acts as a state where the emotional tone is “depotentiated”, also known as the “sleep to remember, sleep to forget” theory. Humans sleep to forget the emotional tone, but still remember the tagged memory of the episode.

In this way, sleep (mainly REM sleep) functions as a way of decoupling the emotional tone from the emotional memories. However, several recent studies are challenging this “depotentiated” theory. For instance, Werner et al. proposed that REM sleep may not work as proposed by Walker, but instead maintain that the emotional tone of memories reinforces the emotional salience of events.

REM sleep deprivation is associated with an increased risk of meeting criteria for PTSD.

Brain development

Sleep is an essential part of our daily routine, with the average person spending about a third of their life asleep. Sleep is vital to brain development, especially in infants and young children.

Newborns sleep between 14 and 18 hours a day, with this decreasing to 12 to 16 hours a day for infants aged 4 to 12 months. During this time, infants spend most of their time in the sleeping state. Sleep is not just a resting state but a period of intense brain activity.

The first year of an infant's life is a time of significant change and development of both the brain and sleep. The control of sleep and the sleep-wake cycle are regulated by the central nervous system (CNS). The maturation of the prenatal brain follows a specific sequence, beginning in the brainstem and progressing to the cerebellum and then the cerebrum.

REM sleep, in particular, is thought to play a vital role in brain development. 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 characterised by muscle relaxation, rapid eye movement, irregular breathing, an elevated heart rate, and increased brain activity. During this stage, the brain processes emotions and consolidates new learnings and motor skills from the day.

From infancy to early childhood, children transition from a biphasic sleep pattern (naps and overnight sleep) to a monophasic sleep pattern (primarily overnight sleep). This transition is suggested to be associated with brain maturation.

During early childhood, the distribution of sleep across the day changes, with children moving from a biphasic sleep pattern to a monophasic sleep pattern. This shift may be due to changes in sleep need, neurocognitive function, and ecological factors.

The need for naps in early childhood has been linked to the more rapid accumulation of sleep pressure in young children, which may be necessary to dissipate. This accumulation of sleep pressure attenuates across early childhood and may explain the transition out of napping during the preschool years.

Sleep EEG characteristics such as sleep spindle density increase until adolescence, while spindle amplitude peaks in the first few years of life and gradually declines across the lifespan. REM sleep is reduced to about 20% of sleep time compared to 50% during infancy.

The need for sleep and sleep patterns change as we age, but this varies across individuals of the same age. Generally, the amount of sleep we need decreases as we get older. School-age children and teens need about 9.5 hours of sleep each night, while most adults require 7 to 9 hours.

Sleep is vital for healthy brain development, with a clear link between sleep patterns and brain maturation. However, it is still unclear whether sleep plays a causal functional role in brain development or merely reflects cortical maturation. Further longitudinal studies are needed to understand the relationship between sleep and the brain fully.

Dreaming vs. non-dreaming sleep

Sleep is divided into four stages. The first two, N1 and N2, are light sleep and typically involve short but often intense non-REM dreams. N3 is deep slow-wave sleep and is also non-REM, with few dreams or none at all. The fourth stage is REM sleep, during which dreams tend to be longer, more expressive, and structured.

During the REM stage, your eyes move rapidly behind your closed eyes, your heart rate speeds up, and your breathing becomes irregular. Your brain is highly active during this stage, and your brain waves become more variable. You experience a temporary loss of muscle tone, except for your eyes, which continue to move rapidly. Researchers believe that this temporary paralysis is a protective measure to stop you from acting out your dreams and injuring yourself. However, this hypothesis is being challenged by the discovery that people can also experience dreams during non-REM sleep.

During non-REM sleep, your eyes don't move, your brain waves are much slower, and you maintain some muscle tone. Your breathing slows down, and your blood pressure and heart rate decrease. In the deeper stages of non-REM sleep, your body repairs and regrows tissues, builds bone and muscle, and strengthens your immune system.

Dreams can happen during any stage of sleep, but the vivid dreams that you remember tend to occur during REM sleep. A 2020 study found that dreams during the REM phase tended to be more elaborate and followed a narrative structure, while dreams during non-REM sleep were more abstract.

Both REM and non-REM sleep are important for different reasons. REM sleep is important for dreaming, memory, emotional processing, and healthy brain development. Non-REM sleep, on the other hand, is when your body repairs and regenerates tissues, builds bone and muscle, and strengthens your immune system.

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