Herbie Murray
Sleep is a natural and reversible state of rest, occurring at regular intervals and necessary for the maintenance of health. It is marked by the absence of wakefulness and a loss of consciousness of one's surroundings. Sleep is accompanied by typical body postures, such as lying down with closed eyes, and is made up of cycles of non-REM and REM sleep. During sleep, the brain exhibits reduced activity and energy consumption, while the body repairs and restores itself. The purpose of sleep is a subject of ongoing research, with recent studies suggesting it may facilitate the removal of waste products from the brain.
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What You'll Learn
Sleep as a natural, reversible state
Sleep is a natural, reversible state experienced by many living organisms. It is characterised by a loss of consciousness and wakefulness, and is accompanied by specific physiological changes, such as a decrease in metabolic rate, reduced responsiveness to external stimuli, and changes in brain activity. Dreaming also occurs during sleep, although the reasons behind it remain a mystery.
Sleep is a highly conserved behaviour across animal evolution, likely dating back hundreds of millions of years. It is promoted by our internal circadian clock, which is influenced by the presence or absence of daylight. Sleep is essential for the restoration and recovery of vital bodily and mental functions. It is during sleep that the brain and body physically restore themselves, with the brain removing metabolic waste products at a faster rate than during wakefulness. This waste removal process is driven by the glymphatic system, which increases the flow of cerebrospinal fluid to carry away metabolites.
Sleep occurs in cycles of non-REM (NREM) and REM sleep. NREM sleep is further divided into four stages, with the final stage, known as delta or slow-wave sleep, characterised by deep sleep. REM sleep, which is marked by increased brain activity, dreaming, and eye movements, occurs as a person returns to a lighter stage of sleep. Each cycle is longer than the one preceding it, as the length of the REM stage increases until waking occurs.
The amount and periodicity of sleep vary with age, with infants sleeping frequently for shorter periods, and mature adults sleeping for longer uninterrupted periods. Sleep is influenced by factors such as age, health, and individual differences, and it is generally recommended that adults aim for around eight hours of sleep per night.
While the exact reasons why we sleep are still being explored, it is clear that sleep is a vital natural process that allows our bodies and minds to recover and function optimally.
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Sleep cycles and stages
Sleep is a highly conserved behaviour across animal evolution, likely going back hundreds of millions of years. Sleep is essential to the restoration and recovery of vital bodily and mental functions. The internal circadian clock promotes sleep daily at night, when it is dark.
The human body cycles through two phases of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM.
Stage 1, or N1, occurs when a person first falls asleep and usually lasts one to seven minutes. During this stage, the body hasn't fully relaxed, but body and brain activities start to slow with periods of brief movements. It is easy to wake someone up during this stage, but if undisturbed, they can quickly move into stage 2. As the night progresses, an uninterrupted sleeper may not spend much more time in stage 1 as they move through further sleep cycles.
During stage 2, or N2, the body enters a more subdued state where body temperature drops, muscles relax, and heart rate and breathing slow. Eye movement stops and brain activity slows, though there are short bursts of activity. Stage 2 can last for 10 to 25 minutes during the first sleep cycle, and each N2 stage can become longer during the night. Collectively, a person typically spends about half of their sleep time in N2 sleep.
Stage 3, also known as N3 or deep sleep, is harder to wake someone up from. Muscle tone, pulse, and breathing rate decrease, and brain activity has an identifiable pattern of delta waves. This stage is critical to restorative sleep, allowing for bodily recovery and growth. It may also bolster the immune system and other key bodily processes.
The duration spent in each sleep stage changes as individuals age, reflecting a decline in the overall biological necessity for sleep over time. Sleep quality and time spent in each sleep stage may also be altered by depression, aging, traumatic brain injuries, medications, and circadian rhythm disorders.
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Sleep disorders
Sleep is a natural, reversible, and periodic state marked by the absence of wakefulness and the loss of consciousness of one's surroundings. It is accompanied by typical body postures, such as lying down with closed eyes, and is essential for the restoration and recovery of vital bodily and mental functions. The internal circadian clock promotes sleep at night, and sleep consists of cycles of non-REM (NREM) and REM sleep.
Insomnia
Insomnia is the most common sleep disorder, characterised by difficulties in falling asleep or staying asleep. To be diagnosed with insomnia disorder, these sleep difficulties must occur at least three nights a week for a minimum of three months, significantly impacting an individual's daily functioning. Treatment for insomnia often involves a combination of sleep medications and behavioural techniques, such as cognitive-behavioural therapy.
Sleep Apnea
Sleep apnea is a breathing disorder where individuals temporarily stop breathing during sleep, sometimes hundreds of times a night. This condition can be treated with a continuous positive airway pressure (CPAP) machine, which helps maintain open airways during sleep.
Restless Leg Syndrome (RLS)
RLS causes an overwhelming urge to move the legs due to uncomfortable sensations. It often occurs alongside sleep apnea and can disrupt the sleep cycle.
Hypersomnia
Hypersomnia is characterised by excessive daytime sleepiness and an inability to stay awake during the day. Narcolepsy falls under this category, causing extreme sleepiness and sudden sleep attacks.
Circadian Rhythm Disorders
These disorders disrupt the sleep-wake cycle, making it difficult for individuals to fall asleep and wake up at the right times. They can be caused by factors such as shift work, jet lag, or changes in sleep schedules.
Parasomnia
Parasomnia involves unusual behaviours during sleep, such as sleepwalking, talking, or eating. It also includes rapid eye movement sleep behaviour disorder, where individuals act out their dreams, potentially causing injury to themselves or their bed partner.
The treatment for sleep disorders varies depending on the specific disorder and can include lifestyle changes, cognitive behavioural therapy, medication, and the use of natural products like melatonin. Sleep disorders are often linked to mental health conditions such as depression and anxiety, and addressing these underlying issues is an important aspect of treatment.
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Sleep and brain activity
Sleep is a natural, reversible, and periodic state marked by the absence of wakefulness and the loss of consciousness of one's surroundings. It is accompanied by changes in brain activity and physiological functioning. The brain uses significantly less energy during sleep, especially during non-REM sleep. The most pronounced physiological changes during sleep occur in the brain.
The brainstem, which consists of the pons, medulla, and midbrain, controls the transitions between wakefulness and sleep. Sleep-promoting cells within the hypothalamus and the brainstem produce a brain chemical called GABA, which reduces activity in the hypothalamus and the brainstem. The brainstem, especially the pons and medulla, also plays a crucial role in REM sleep, sending signals to relax muscles and prevent us from acting out our dreams.
The basal forebrain, located near the front and bottom of the brain, promotes sleep and wakefulness. The release of a chemical called adenosine from cells helps induce sleepiness. Adenosine levels increase during prolonged wakefulness and decrease during sleep, potentially acting as a homeostatic regulator of sleep. Caffeine blocks the action of adenosine, counteracting sleepiness.
The sleeping brain removes metabolic waste products at a faster rate than when awake, through the glymphatic system, which increases the flow of cerebrospinal fluid. Sleep may facilitate the synthesis of molecules that protect the brain from metabolic end products generated during wakefulness. Anabolic hormones, such as growth hormones, are secreted during sleep.
The internal circadian clock, regulated by the "master clock" of the brain, the suprachiasmatic nucleus, promotes sleep daily at night when it is dark. Circadian rhythms, or near 24-hour biological rhythms, are genetically regulated and influence cellular and molecular processes in the body. Sleep and circadian rhythms play a vital role in brain health, and their improvement may help prevent and treat brain diseases.
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Sleep and health
Sleep is a natural and easily reversible state marked by the absence of wakefulness and the loss of consciousness of one's surroundings. It is usually accompanied by a typical body posture, such as lying down with the eyes closed, and the occurrence of dreaming. Sleep is essential for the restoration and recovery of vital bodily and mental functions.
Good sleep is crucial for maintaining good health and emotional well-being. The amount of sleep one needs changes with age, but on average, most adults require at least seven to nine hours of uninterrupted and refreshing sleep per night. Getting a good night's sleep can help improve heart health, metabolism, and weight management. It also reduces the risk of developing medical conditions such as obesity, diabetes, and cardiovascular disease.
Sleep plays a vital role in maintaining healthy blood sugar levels. Studies have shown that individuals who consistently sleep fewer than five hours per night have an increased risk of developing type 2 diabetes. Conversely, improved sleep quality can positively influence blood sugar control and reduce the impact of type 2 diabetes.
Additionally, sleep has a significant impact on the immune system. Sleep deprivation increases inflammatory mediators in the body, making individuals more susceptible to infections and illnesses, such as the common cold. Those who get adequate and better-quality sleep tend to have stronger immune systems and are less likely to fall sick.
Sleep disorders, such as insomnia, sleep apnea, and restless leg syndrome, can affect an individual's ability to get quality sleep. These disorders can have various causes, including underlying medical conditions, stress, or environmental factors. It is important to seek help from a healthcare provider if you regularly experience sleep difficulties, as they can help identify any underlying issues and provide guidance on improving sleep habits and managing stress.
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Frequently asked questions
Sleep is a natural, reversible state of rest in most vertebrate animals, occurring at regular intervals and necessary for the maintenance of health. It is marked by the absence of wakefulness and the loss of consciousness of one's surroundings. Sleep is accompanied by a typical body posture (such as lying down with the eyes closed), the occurrence of dreaming, and changes in brain activity and physiological functioning.
Our bodies require sleep in order to maintain proper function and health. Sleep is considered essential to the restoration and recovery of vital bodily and mental functions. Sleep may also facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking.
Sleep occurs in cycles of non-REM (NREM) sleep and REM sleep. During non-REM sleep, brain activity decreases while the body recovers from wakeful activity. During REM sleep, dreams occur, and memory is thought to be organized. The brain uses significantly less energy during sleep, especially during non-REM sleep.
The internal circadian clock promotes sleep daily at night, when it is dark. The use of artificial light has substantially altered humanity's sleep patterns, with sources such as digital device screens emitting large amounts of blue light. Caffeine consumption can also impact sleep, with reduced intake potentially improving sleep quality.
