Brain Activity: Rem Sleep Vs Waking

Sleep is a complex and mysterious body process that remains an active area of research. On average, humans spend about one-third of their lives asleep, cycling between two types of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. REM sleep is distinguished from waking by several unique characteristics. During REM sleep, the eyes move rapidly in different directions, and brain activity is similar to its activity when awake. Dreams typically occur during this stage. In contrast, during NREM sleep, the brain is less active, and in the deeper stages, breathing slows down and blood pressure drops. After falling asleep, individuals typically enter NREM sleep first, followed by a shorter period of REM sleep, and then the cycle repeats.

Brain activity

During the first stage of non-REM sleep, the lightest stage of sleep, brain activity transitions from wakefulness (marked by rhythmic alpha waves) to low-voltage, mixed-frequency waves. This stage lasts around 1 to 5 minutes, comprising 5% of total sleep time.

The second stage of non-REM sleep is deeper sleep, during which the heart rate and body temperature drop. This stage lasts around 25 minutes in the first cycle and lengthens with each successive cycle, eventually comprising about 45% of total sleep. This stage is characterised by sleep spindles and K-complexes, which are believed to be important for memory consolidation.

The third stage of non-REM sleep is the deepest and is characterised by slow-wave activity. This is the most difficult stage to wake someone up from, and if they are awoken, they will likely experience sleep inertia, a state of confusion or mental fog that can last up to 30 minutes. This stage makes up about 25% of total sleep time in adults, but the percentage decreases with age.

During REM sleep, the brain is highly active, with brain metabolism increasing by up to 20%. Brain activity during this stage is similar to that of someone who is awake. Dreaming mostly occurs during REM sleep, and it is believed that the brain processes emotions and transfers short-term memories into long-term memories during this stage. The first cycle of REM sleep typically lasts about 10 minutes, with each subsequent cycle getting longer, up to an hour for the final cycle. REM sleep makes up about 20-25% of total sleep time.

Eye movement

During REM sleep, the eyes move rapidly behind closed eyelids. This is one of the unique characteristics of this stage of sleep, alongside increased brain activity, irregular breathing, elevated heart rate, and relaxed muscles. The first cycle of REM sleep occurs about 60-90 minutes after falling asleep, and the first period typically lasts around 10 minutes. Each subsequent REM cycle gets longer, with the final one lasting up to an hour.

REM sleep is important for several reasons. It plays a role in memory consolidation, emotional processing, brain development, and dreaming. Dreaming mostly takes place during REM sleep, and the dreams are usually more vivid than those that occur during non-REM sleep. The brain also processes emotions during this stage, and the amygdala (the part of the brain responsible for processing emotions) is activated.

The amount of REM sleep needed changes throughout our lives. Newborn babies spend eight hours in REM sleep per day, while adults only need around two hours per night on average.

Muscle tone

Dreaming occurs during REM sleep, and this stage of sleep is characterised by changes in brain signalling that cause reduced muscle tone in many of the body's muscles. This is known as REM sleep muscle paralysis or muscle atonia and is considered a normal function of REM sleep.

During REM sleep, the body experiences a temporary loss of muscle tone, except for the eyes, which move rapidly. 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 when the body is not paralysed.

REM sleep muscle paralysis is caused by a suppression of postural muscle tone. This is due to the inactivation of somatic motoneurons, which keeps the skeletal motor system quiescent during REM sleep.

The loss of muscle tone during REM sleep is not absolute. People with REM sleep behaviour disorder (RBD) experience increased muscle tone during this stage of sleep, allowing them to move and act out their dreams. This can range from minor movements like leg twitches to very complex behaviour that may result in injury to the individual or their bed partner. RBD is often associated with certain neurological disorders and is more common with age.

Heart rate

During REM sleep, the heart rate speeds up and becomes more variable. This is in contrast to non-REM sleep, during which the heart rate slows down.

REM sleep is characterised by a heart rate that is more similar to that of a waking person than non-REM sleep. During non-REM sleep, the heart rate is at its slowest of the night.

Each sleep cycle takes 90 to 120 minutes, and the first REM cycle is the shortest, at around 10 minutes. Each subsequent REM cycle gets longer, with the final one lasting up to an hour.

The heart rate is also affected by the stages of non-REM sleep. During the first stage of non-REM sleep, the heart rate is regular. In the second stage, the heart rate slows and the body temperature drops. The third stage of non-REM sleep is deep sleep, during which the heart rate is at its slowest and it is difficult to wake the sleeper.

The heart rate is also affected by age, with older adults tending to experience earlier sleep and wake times, and reduced sleep consolidation.

Breathing

During REM sleep, the breathing rate is erratic and irregular, and the skeletal muscles are atonic and without movement, except for the eyes and diaphragmatic muscles, which remain active. This is in contrast to non-REM sleep, where breathing is regular and slow.

The respiratory system is also affected by changes in blood pressure and heart rate during sleep, which are primarily determined by autonomic nervous system activity. Brief increases in blood pressure and heart rate occur with K-complexes, arousals, and large body movements. There is also an increased risk of myocardial infarction in the morning due to sharp increases in heart rate and blood pressure upon awakening.

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