Brain Activity And Rem Sleep: What's Missing?

REM sleep, or rapid eye movement sleep, is the fourth of four stages of sleep. During this stage, the eyes move rapidly, and brain activity increases. However, the body experiences temporary muscle paralysis, except for the eyes and muscles required for breathing. This stage is also associated with dreaming and memory consolidation. While REM sleep is crucial for overall health, what is absent during this stage of sleep is muscle movement and control.

Muscle movement

During REM sleep, the body experiences a temporary loss of muscle tone, and skeletal muscles are atonic and without movement. This is thought to be a protective measure to prevent sleepers from acting out their dreams and injuring themselves. However, this hypothesis is being challenged by the discovery that dreams can also occur during non-REM sleep.

REM sleep is also associated with irregular muscle movements, such as sudden limb movements. This is known as REM sleep disorder, which may be caused by degenerative neurological conditions such as Parkinson's disease or Lewy body dementia, or the use of antidepressants.

Narcolepsy is another sleep disorder that affects REM sleep. Individuals with narcolepsy tend to skip the initial phases of sleep and fall directly into REM sleep. They experience brief episodes of muscle weakness (cataplexy) and can enter the REM phase during short naps, limiting their time in the deep-sleep stage and causing an irregular sleep pattern.

Sleepwalking, or somnambulism, is also associated with REM sleep. It typically occurs in school-aged children whose sleep cycles are still maturing. Sleepwalking happens during the non-rapid eye movement phases, usually during N3 sleep.

Brain activity

During REM sleep, the brain's metabolism increases by up to 20%, and it consumes more oxygen. Brain wave recordings during this stage show patterns similar to those seen when a person is awake, with beta waves being the most prevalent. These brain waves indicate that the brain remains highly active, even as the body experiences temporary muscle paralysis and erratic breathing patterns.

The role of REM sleep in memory consolidation is particularly noteworthy. This is when the brain processes new learnings and skills acquired during the day, deciding what to commit to long-term memory and what to delete. Studies have shown that deprivation of REM sleep can interfere with memory formation, although it is not yet clear if this is solely due to the loss of REM sleep or overall sleep disruption.

Emotional processing is another key aspect of REM sleep. The amygdala, the part of the brain responsible for processing emotions, is activated during this stage. Dreams, which tend to be more vivid during REM sleep, may also play a role in emotional processing.

Additionally, REM sleep is crucial for brain development, especially in infants and children. Newborns spend a significant portion of their sleep in the REM stage, and the duration of REM sleep gradually decreases as we age. This suggests that REM sleep plays a vital role in the developing brain, with animal studies further supporting this idea.

Furthermore, REM sleep may contribute to wakefulness preparation. As the night progresses, we spend increasing amounts of time in REM sleep, which may help explain why we are easier to wake up during this stage.

Eye movement

During REM sleep, the eyes move rapidly behind closed eyelids. This is where the name 'rapid eye movement sleep' comes from. The eyes move more slowly than they do when we are awake, and the movements tend to be shorter and more likely to loop back to their starting point. There are about seven of these loops per minute of REM sleep.

The purpose of these eye movements has long been a mystery, with many researchers writing them off as random actions, perhaps to keep the eyelids lubricated. However, a 2022 study by researchers at the University of California, San Francisco, found that eye movements during REM sleep are coordinated with what is happening in the dream world created by the brain. In other words, when our eyes move during REM sleep, we are gazing at things in the dream.

The scanning hypothesis proposes that the directional properties of REM sleep are related to a shift of gaze in dream imagery. However, this theory is challenged by the fact that eye movements occur in those born blind and in fetuses, despite a lack of vision.

REM sleep is also characterised by irregular breathing, an elevated heart rate, and increased brain activity.

Heart rate

During REM sleep, your heart rate speeds up and becomes more similar to your heart rate when you are awake. In contrast, during non-REM sleep, your heart rate slows down.

During the first stage of non-REM sleep, your heart rate begins to slow down. In the second stage of non-REM sleep, your heart rate and body temperature decrease further as your body prepares for deep sleep. The third stage of non-REM sleep is deep sleep, during which it is difficult to wake someone up. If they are woken up during this stage, they will likely experience sleep inertia, a state of confusion or mental fog that lasts for about 30 minutes.

During REM sleep, your heart rate can vary depending on the activity level in your dream. For example, if you are running in your dream, your heart rate will rise as if you were awake. Researchers believe that the surge in activity during REM sleep could explain why vulnerable people often experience heart attacks and other cardiovascular events in the early morning hours, when REM sleep is more prevalent.

On average, an adult's resting heart rate is between 60 and 100 beats per minute (bpm). During sleep, it is normal for a person's heart rate to drop to between 40 and 50 bpm. However, there is variability between individuals, and factors such as age, stress, anxiety, sleep behaviours, and pregnancy can influence heart rate during sleep.

Overall, while heart rate does not become "absent" during REM sleep, it does fluctuate throughout the different stages of sleep, with REM sleep typically being associated with a faster heart rate compared to non-REM sleep.

Breathing

During REM sleep, breathing becomes irregular. While the body operates similarly to how it does when awake, the breathing rate is more erratic and unpredictable.

REM sleep is the fourth and final stage of sleep, and it is characterised by irregular breathing, rapid eye movement, relaxed muscles, an elevated heart rate, and increased brain activity. It is also known as active sleep, desynchronized sleep, paradoxical sleep, rhombencephalic sleep, and dream sleep.

During the REM stage, the body is temporarily paralysed, except for the eyes and the muscles that control breathing. This is thought to be a protective measure to stop people from acting out their dreams and injuring themselves. However, this hypothesis is being challenged by the discovery that dreams can also occur during non-REM sleep.

The first cycle of REM sleep occurs around 60-90 minutes after falling asleep, and it usually lasts about 10 minutes. As the night progresses, the REM cycles get longer, with the final one lasting up to an hour.

Babies and children require more REM sleep than adults. Newborns spend eight hours in REM sleep each day, while adults only need an average of two hours per night.

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