Exploring Rem Sleep: Unlocking The Brain's Physiological Mysteries

During REM sleep, the body undergoes a series of physiological changes. Brain activity shoots back up to levels similar to when a person is awake, which is why REM sleep is associated with the most intense dreams. While breathing and heart rate increase during REM sleep, most muscles are paralysed, preventing people from acting out their dreams. This is known as atonia.

REM sleep is also characterised by theta waves in the hippocampus, which is associated with processing and storing memories. The eyes move in many directions while closed, and researchers think that eye movement direction correlates with actions occurring in dreams.

Other physiological changes that occur during REM sleep include temperature fluctuation, elevated oxygen consumption, increased levels of acetylcholine, irregular breathing, and fluctuations in blood pressure.

Brain activity

During REM sleep, the brain exhibits a unique pattern of activity that is distinct from the other stages of sleep. This stage is characterised by low-voltage, mixed-frequency brain waves, which are similar to the brain waves observed during wakefulness. This has led to REM sleep sometimes being referred to as "paradoxical sleep".

The brain activity during REM sleep is characterised by:

• Low-amplitude, high-frequency beta waves
• High-amplitude rhythmic brain waves called theta waves, which are generated in the hippocampus and are associated with memory and spatial processing
• "Sawtooth" waveforms
• Theta activity (3 to 7 counts per second)
• Slow alpha activity

During REM sleep, the brain selectively suppresses responses to external stimuli. However, certain stimuli, such as the sound of a crying infant, can still trigger a response and wake a person from sleep.

The purpose and function of REM sleep are not yet fully understood. Some theories suggest that it plays a role in brain development, memory consolidation, and learning. The changes in brain activity during this stage support these theories. For example, the activation of the hippocampus and amygdala during REM sleep may help regulate memory and emotion, while the thalamus blocks the processing of sensory information from the environment.

Eye movement

During REM sleep, the eyes move in many directions while closed. This is where the name "rapid eye movement sleep" comes from. The directionality of movement is irregular and unpredictable. However, researchers think that eye movement direction correlates with actions occurring in dreams. This has been found to be true in mice, in which eye movements and the activity of brain cells that perceive head orientation were monitored simultaneously. Although the mice were immobilized and sleeping, head orientation cell activation predicted eye movement direction.

REM sleep eye movements also activate the visual cortex of the brain, which does not occur when individuals who are awake close their eyes and make eye movements. The extraocular movements after which REM sleep is named are largely due to the cholinergic activity of the pedunculopontine tegmentum (PPT) center and glutamatergic activity of the medial pontine reticular formation (mPRF).

Muscle tone

During REM sleep, the body experiences muscle atonia, a temporary paralysis of the skeletal muscles. This keeps the body from acting out dreams. However, the muscles required for breathing and eye movement remain active. The direction of eye movement during REM sleep is thought to correlate with the actions occurring in dreams.

REM sleep is also associated with increased oxygen consumption, elevated levels of acetylcholine, irregular breathing, and fluctuations in blood pressure.

Heart rate

During REM sleep, the heart rate increases to a level similar to that of when a person is awake. This is in contrast to the earlier stages of sleep, in which the heart rate slows down. During the first stage of non-rapid eye movement (NREM) sleep, the heart rate begins to slow, reaching its lowest pace during the third stage of NREM sleep.

The increase in heart rate during REM sleep is accompanied by an increase in blood pressure. This can put individuals at risk of myocardial infarction in the morning due to sharp increases in both heart rate and blood pressure when waking up.

The heart rate is also affected by the circadian rhythm, which controls the sleep-wake cycle and regulates the body's temperature, muscle tone, and hormone secretion. The circadian rhythm is generated by neural structures in the hypothalamus and acts as a biological clock.

Breathing

During REM sleep, breathing may become irregular and increase in rate. In non-REM sleep, respiration reaches its lowest rate during the deep sleep stage three.

Respiratory changes during sleep are complex and vary between REM and non-REM sleep. Ventilation and respiratory flow become faster and more erratic during REM sleep, and hypoventilation may occur in a similar way to non-REM sleep. Hypoventilation is caused by reduced pharyngeal muscle tone, reduced rib cage movement, and increased upper airway resistance due to loss of muscle tone in the intercostals and upper airway muscles. The cough reflex is also suppressed during both REM and non-REM sleep.

The hypoxic ventilatory response is lower in non-REM sleep than during wakefulness and decreases further during REM sleep. The arousal response to respiratory resistance is lowest in non-REM sleep stages three and four.

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