Herbie Murray
Sleep is a vital process that allows the body to rest, repair, and restore itself. A typical night's sleep consists of 4 to 5 sleep cycles, with each cycle lasting around 90 minutes and progressing through various stages of sleep. These stages include non-rapid eye movement (NREM) sleep, divided into three substages (N1, N2, and N3), and rapid eye movement (REM) sleep. Each stage is associated with distinct physiological changes and plays a specific role in the body's recovery and growth processes. Understanding these sleep stages and their significance is essential for maintaining optimal sleep hygiene and overall health.
| Characteristics | Values |
|---|---|
| Number of sleep stages | 4 |
| Number of sleep phases | 2 |
| First sleep phase | Non-rapid eye movement (NREM) or non-REM |
| Number of NREM stages | 3 |
| NREM stages | N1, N2, N3 |
| N1 | Easy to wake up; quick progression to N2 if uninterrupted |
| N2 | Body temperature drops; muscles relax; heart rate and breathing slow; eye movement stops; brain activity slows |
| N3 | Deep sleep or slow-wave sleep; brain waves are slow but strong; body repairs injuries and reinforces immune system; difficult to wake up from |
| Second sleep phase | Rapid eye movement (REM) |
| REM sleep | Eyes twitch; brain is active and similar to during waking hours; breath rate increases; body is temporarily paralysed; dreaming occurs |
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What You'll Learn
Sleep is restorative
Sleep is one of the most important activities for your health. A healthy amount of sleep is vital for "brain plasticity", or the brain's ability to adapt to input. If we sleep too little, we become unable to process what we've learned during the day and have more trouble remembering it in the future. Researchers also believe that sleep may promote the removal of waste products from brain cells, which seems to occur less efficiently when the brain is awake. Sleep is vital for the rest of the body, too. When people don't get enough sleep, their health risks rise, with symptoms of depression, seizures, high blood pressure, and migraines worsening. Sleep also impacts the immune system, with immunity compromised by a lack of sleep, increasing the likelihood of illness and infection.
Stage 3 NREM sleep is when the body performs various health-promoting functions. This is the deepest sleep stage, where brain waves are slow but strong. The body takes advantage of this very deep sleep stage to repair injuries and reinforce the immune system. During this stage, the same bursts of brain activity that happen in stage 2 can also occur, and brain waves specific to stage 3 help regulate those bursts. You need stage 3 NREM sleep to wake up feeling rested. Without enough stage 3 sleep, you will feel tired and drained, even if you slept for a long time. That's why the body automatically tries to get as much stage 3 sleep as early as possible during the sleep period.
After stage 3 NREM sleep, the body moves into stage 2 NREM, which is the gatekeeper of REM sleep. During stage 2, the body enters a more subdued state where body temperature drops, muscles relax, and heart rate and breathing slow. At the same time, eye movement stops and brain activity slows, though there are short bursts of activity that help resist being woken up by external stimuli. Stage 2 sleep can last for 10 to 25 minutes during the first sleep cycle, and each N2 stage can become longer during the night.
During REM sleep, the eyes move rapidly behind closed lids, and brain waves are similar to those during wakefulness. Breath rate increases, and the body becomes temporarily paralyzed as we dream. The cycle then repeats itself, but with each cycle, you spend less time in the deeper stages three and four of sleep and more time in REM sleep.
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Sleep stages vary with age
Sleep is a body process that allows the body to rest, repair, and restore itself. A typical night of healthy sleep consists of four stages, three of which are non-rapid eye movement (NREM) sleep stages, and the fourth being the rapid eye movement (REM) sleep stage. The four stages are:
- N1: Occurs when a person first falls asleep and usually lasts one to seven minutes. The body hasn't fully relaxed, but body and brain activities start to slow with brief movements.
- N2: The body enters a more subdued state where body temperature drops, muscles relax, and heart rate and breathing slow.
- N3: Deep sleep or slow-wave sleep, where the brain exhibits a particular pattern of brain activity. This is the most restorative stage of sleep, allowing the body to recover and grow.
- REM Sleep: Brain activity increases, the body becomes temporarily paralyzed, and dreaming occurs.
Sleep cycles can vary from person to person and are influenced by factors such as age, recent sleep patterns, and alcohol consumption. Indeed, sleep stages vary with age. Newborns spend more time in REM sleep and may enter this stage as soon as they fall asleep. As they get older, their sleep patterns gradually start to resemble those of adults. The amount of slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. Older adults tend to spend less time in REM sleep.
The duration of sleep also changes with age. Infants typically sleep 14 to 15 hours daily, with most sleep occurring in the evening and only 1 to 2 naps needed during the day. Around 2 to 5 years of age, the total sleep time needed each day decreases by 2 hours, from 13 to 11 hours. By 6 years of age, children manifest circadian sleep phase preferences and tend to be night owls or early risers. Adolescents require 9 to 10 hours of sleep each night.
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Sleep disorders
Insomnia
The most common sleep disorder, insomnia is characterised by the inability to fall asleep and stay asleep.
Sleep apnea
A breathing disorder where breathing stops for 10 seconds or more during sleep. Sleep apnea can be treated with a CPAP (continuous positive airway pressure) machine.
Restless leg syndrome (RLS)
People with RLS experience a tingling or prickly sensation in their legs, along with a powerful urge to move them.
Hypersomnia
People with hypersomnia are unable to stay awake during the day. This includes narcolepsy, which causes extreme daytime sleepiness.
Circadian rhythm disorders
Circadian rhythm disorders are problems with the sleep-wake cycle, making it difficult to fall asleep and wake up at the right times.
Parasomnia
People with parasomnia act in unusual ways while falling asleep, sleeping, or waking up, such as walking, talking, or eating.
The treatment for sleep disorders depends on the specific disorder. Some common treatments include:
- Good sleep habits and lifestyle changes, such as maintaining a healthy weight, exercising regularly, and limiting naps.
- Cognitive-behavioural therapy (CBT) and relaxation techniques to reduce anxiety and improve sleep habits.
- Medication, including sleeping pills and natural products like melatonin.
- Creating a safe and comfortable sleeping environment, such as keeping the bedroom dark, cool, and free of hazards.
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Sleep architecture
The first stage of sleep is N1, where it is easy to wake someone up. If undisturbed, the person quickly moves on to the next stage. During N2, the body enters a subdued state, with a drop in body temperature, muscle relaxation, and slower heart rate, breathing, and eye movement. Brain activity also slows down, although short bursts of activity help the sleeper resist external stimuli. N2 sleep lengthens with each sleep cycle, eventually comprising about 45% of total sleep.
The final stage of NREM sleep is N3, also known as slow-wave sleep (SWS). This is the deepest stage of sleep, characterised by slow yet strong brain waves. The body takes advantage of this stage to repair injuries and reinforce the immune system. N3 is the most difficult stage to wake someone up from, and if they do wake up, they will likely experience sleep inertia, a state of confusion or mental fog.
After N3, the body moves back to N2, which is the gatekeeper of REM sleep. During REM sleep, the eyes move rapidly, and brain waves are similar to those during wakefulness. The breath rate increases, and the body becomes temporarily paralysed as the sleeper dreams. Dreaming usually occurs during this stage, and the muscles become limp to prevent the sleeper from acting out their dreams. Each sleep cycle, the sleeper spends less time in the deeper stages of sleep and more time in REM sleep.
On a typical night, a person goes through four to six sleep cycles, each lasting around 90 minutes. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes, while later cycles tend to be longer, ranging from 90 to 120 minutes. Sleep cycles can vary depending on various factors such as age, recent sleep patterns, and alcohol consumption. For example, alcohol decreases REM sleep early in the night, but as it wears off, there is a REM sleep rebound with prolonged REM stages. Sleep disorders such as sleep apnea and restless leg syndrome can also interrupt the healthy sleep cycle.
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Sleep cycles
Sleep is divided into two phases: rapid-eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further divided into three stages, from N1 to N3.
During the first stage of sleep, N1, it is easy to wake someone up, but if they are not disturbed, they can quickly move into stage N2. As the night progresses, an uninterrupted sleeper may not spend much more time in stage N1, moving through further sleep cycles.
During stage N2, the body enters a 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 that help resist being woken up by external stimuli. Stage N2 sleep can last for 10 to 25 minutes during the first sleep cycle, and each stage can become longer during the night.
Stage N3 is the deepest sleep stage. Brain waves are slow but strong, and the body takes advantage of this very deep sleep stage to repair injuries and reinforce the immune system. The same bursts of brain activity that happen in stage N2 can also happen in stage N3, and brain waves specific to this stage help regulate those bursts. You need stage N3 sleep to wake up feeling rested. Without enough sleep at this stage, you will feel tired and drained even if you slept for a long time. That is why the body automatically tries to get as much stage N3 sleep as early as possible. After stage N3, the body moves back into stage N2, which is the gatekeeper of REM sleep. It is hard to wake someone up from stage N3 sleep, and if they do wake up, they will probably experience sleep inertia, a state of confusion or mental fog that lasts about 30 minutes.
During REM sleep, the eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. The breath rate increases, and the body becomes temporarily paralysed as we dream. Dreaming usually happens during REM sleep, and the muscles become limp to prevent sleepers from acting out their dreams. People usually have more REM sleep later in the night, but they do not have as much REM sleep in colder temperatures. This is because the body does not regulate its temperature properly during REM sleep.
On a typical night, a person goes through four to six sleep cycles. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes, while later cycles tend to fall between 90 and 120 minutes. Sleep cycles can vary from person to person and from night to night based on a wide range of factors, such as age, recent sleep patterns, and alcohol consumption.
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