Brain Activity During Rem Sleep: An Intriguing Mystery

Sleep is a complex and dynamic process that affects our functioning in ways that scientists are only beginning to understand. While we sleep, the brain remains remarkably active, and recent findings suggest that sleep plays a housekeeping role, removing toxins that build up while we are awake.

There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep. Dreaming mostly occurs during REM sleep, when the brain is most active. During REM sleep, the brain's electrical activity, as measured by an electroencephalogram (EEG), is similar to that of a waking brain. However, the body's muscles are temporarily paralysed, except for those that enable breathing and control eye movements.

The purpose of REM sleep remains a biological mystery, despite growing understanding of its biochemistry and neurobiology.

Brain activity during REM sleep

Brain activity during sleep is vital for brain health and for solidifying memories. While sleeping, the brain is far from dormant and remains remarkably active.

During the first hour of sleep, brain waves slow down, and the eyes and muscles relax. Heart rate, blood pressure, and body temperature fall as well. However, over the next half hour, brain activity drastically increases from slow-wave sleep to rapid eye movement (REM) sleep. Brain activity during REM sleep looks very similar to that of a waking brain.

REM sleep is bizarre: a sleeper's brain becomes highly active while their body's muscles are paralysed, and their breathing and heart rate become erratic. The purpose of REM sleep remains a biological mystery, despite our growing understanding of its biochemistry and neurobiology. We know that a small group of cells in the brain stem, called the subcoeruleus nucleus, controls REM sleep.

During REM sleep, signals from the pons travel to the thalamus, which relays them to the cerebral cortex, stimulating its regions responsible for learning, thinking, and organising information. Some scientists believe dreams are the cerebral cortex's attempt to "find meaning in the random signals that it receives during REM sleep". Essentially, the cortex may be trying to interpret these random signals, "creating a story out of fragmented brain activity".

REM sleep makes up about 20% of our total sleep and typically occurs 90 minutes after falling asleep. Most of our dreaming occurs during this stage, although some can also happen during non-REM sleep. As sleep continues, the brain alternates between periods of slow-wave sleep and brief periods of REM sleep, with the slow waves becoming less deep and the REM periods more prolonged until waking.

Dreaming

During REM sleep, the brain becomes highly active while the body's muscles are temporarily paralysed, and breathing and heart rate become erratic. This stage of sleep remains a biological mystery, despite growing understanding of its biochemistry and neurobiology. While the purpose of REM sleep is not fully understood, one hypothesis suggests that it may be the brain's way of recovering consciousness from the disruption of deep sleep.

REM sleep typically occurs 90 minutes after falling asleep and makes up about 20% of our total sleep. It is characterised by rapid eye movements, and the brain activity observed during this stage resembles that of a waking brain. However, unlike when we are awake, our bodies experience atonia, or muscle paralysis, during REM sleep. The only muscles that remain active are those that control eye movements and enable breathing. As we age, we spend less of our time in REM sleep.

While the exact purpose of dreaming is not known, it is speculated that it may help in processing emotions, with events from the day often invading our thoughts during sleep. People suffering from stress or anxiety are more likely to have frightening dreams. Dreams can occur during all stages of sleep but are usually most vivid in REM sleep. Some people dream in colour, while others recall dreams only in black and white.

Sleep cycles

Sleep is a complex and dynamic process that affects our functioning in ways scientists are only beginning to understand. Sleep cycles are an important part of this process.

There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep. Within non-REM sleep, scientists have identified three different stages. Each is linked to specific brain waves and neuronal activity.

During the first hour of sleep, brain waves slow down, and the eyes and muscles relax. Heart rate, blood pressure, and body temperature fall as well. However, over the next half hour, brain activity drastically increases from slow-wave sleep to REM sleep, and brain waves observed during REM are similar to those observed during wakefulness.

During REM sleep, the brain is highly active while the body's muscles are paralysed, and breathing and heart rate become erratic. The purpose of REM sleep remains a biological mystery, despite our growing understanding of its biochemistry and neurobiology. We do know that it is controlled by a small group of cells in the brain stem, called the subcoeruleus nucleus.

During a typical night, we cycle through non-REM and REM sleep several times, with increasingly longer and deeper REM periods occurring later in the sleep session. One cycle normally takes about 90 to 120 minutes. Most people go through four or five cycles per night (assuming they get a full eight hours of sleep).

Stage 1 non-REM sleep is the lightest stage of sleep and usually lasts only a few minutes, making up about 5% of your sleep time. After that, your sleep gets deeper, and you move into stage 2 non-REM sleep. Stage 2 non-REM sleep accounts for about 45% of your time asleep and is characterised by slow and powerful bursts of electrical activity. Experts think that these bursts are your brain organising memories and information from the time you spent awake.

The deepest stage of non-REM sleep is stage 3. It makes up about 25% of your total sleep time in adults but is more common in babies and children. In this stage, your brain waves are slow but strong, and your body takes advantage of this very deep sleep stage to repair injuries and reinforce your immune system.

REM sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids, and brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near-waking levels. Most of your dreaming occurs during REM sleep, and your arm and leg muscles become temporarily paralysed, which prevents you from acting out your dreams. As you age, you spend less of your time in REM sleep.

In summary, while we sleep, our brains cycle through distinct stages of non-REM and REM sleep. Each stage has unique characteristics and plays a vital role in our overall health and well-being.

Sleep disorders

• Central disorders of hypersomnolence (e.g. narcolepsy)
• Circadian rhythm sleep-wake disorders (e.g. jet lag, shift work sleep disorder)
• Sleep-disordered breathing (e.g. sleep apnea)
• Sleep-related movement disorders (e.g. restless leg syndrome)
• Parasomnias (e.g. sleep myoclonus, exploding head syndrome, bruxism, etc.)
• REM sleep behavior disorder, narcolepsy, and nightmare disorder

REM Sleep Behavior Disorder (RBD)

People with RBD do not experience muscle paralysis during REM sleep, allowing them to act out their dreams. They may shout, punch, kick, or jerk in their sleep, potentially injuring themselves or their sleep partner. RBD is often caused by a breakdown in the brainstem, specifically the area responsible for regulating REM sleep. It frequently precedes the development of neurodegenerative diseases.

Narcolepsy

People with narcolepsy experience sudden loss of muscle tone while awake, known as cataplexy, as they instantly fall into REM sleep. They also suffer from excessive daytime sleepiness, disrupted REM sleep, and hypnagogia, or dream-like hallucinations before falling asleep. Narcolepsy appears to result from a loss of orexin neurons in the hypothalamus.

Nightmare Disorder

Nightmare disorder is characterised by frequent and distressing nightmares. It is often triggered by stress, childhood trauma, or other frightening experiences.

Obstructive Sleep Apnea (OSA) and Central Sleep Apnea (CSA)

OSA and CSA are sleep apnea disorders that affect the amount of REM sleep a person obtains. People with sleep apnea experience lapses in breathing during sleep, often moving to a lighter sleep cycle to continue breathing. Consequently, they spend less time in REM sleep and experience excessive daytime sleepiness.

Brain maintenance

Sleep is an essential part of our lives, and while we sleep, the brain is far from dormant. Brain activity during sleep is vital for brain health and for solidifying memories. During sleep, the brain reorganises and catalogues memories and learned information, making it easier to access and use learned information.

The brain generates two distinct types of sleep—slow-wave sleep (SWS) and rapid eye movement (REM) sleep. Most of our sleep is of the SWS variety, which is characterised by large, slow brain waves, relaxed muscles, and slow, deep breathing, which may help the brain and body recuperate after a long day.

During the first hour of sleep, brain waves slow down, and the eyes and muscles relax. Heart rate, blood pressure, and temperature fall as well. However, over the next half hour, brain activity drastically increases from slow-wave sleep to REM sleep, and brain waves observed during REM are similar to those observed during wakefulness. During REM sleep, the brain is highly active, while the body's muscles are paralysed, and breathing and heart rate become erratic.

REM sleep is a biological mystery, but it is known that a small group of cells in the brain stem, called the subcoeruleus nucleus, controls it. When these cells become injured or diseased, people do not experience the muscle paralysis associated with REM sleep, which can lead to REM sleep behaviour disorder—a serious condition in which the afflicted violently act out their dreams.

During REM sleep, signals from the pons travel to the thalamus, which relays them to the cerebral cortex, the outer layer of the brain, and stimulate its regions responsible for learning, thinking, and organising information. Some scientists believe dreams are the cerebral cortex's attempt to "find meaning in the random signals that it receives during REM sleep". Essentially, the cortex may be trying to interpret these random signals, "creating a story out of fragmented brain activity".

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