What are the stages of sleep?
When you are asleep, your eyes are closed, most of your muscles are relaxed and your consciousness is practically suspended. While your body is mostly still, your brain is quite active.
Your body’s circadian rhythm (or, internal biological ‘clock’) control when you feel sleepy and when you’re typically awake. If this system is disrupted — such as due to shift work or jet lag — you can experience sleep problems.
Sleep has 2 main phases:
rapid eye movement (REM) sleep
non-rapid eye movement (non-REM) sleep
When you are in non-REM sleep, you go through 4 stages. During stage 1, you are transitioning between being awake and asleep, and wake easily. During stages 2, 3 and 4, your eye movements stop, your body temperature drops and you are deeply asleep.
In REM sleep, your eyes move rapidly, your blood pressure and heart rate go up, and your brain becomes very active. REM sleep is when most dreaming occurs and is thought to be important for learning and creating new memories.
REM sleep happens about every 90 minutes during the night. Adults usually spend about one-fifth of the night in REM sleep and the remaining four-fifths in non-REM sleep. Babies spend at least half of the time they’re asleep in REM sleep.
Not getting enough sleep can seriously affect a person’s health. As well as impacting concentration and mood, lack of sleep has been linked to a range of conditions such as heart disease, high blood pressure, diabetes and even premature death.
In the short term, people who don’t get adequate sleep are more likely than others to have road accidents. It can also lead to problems with productivity and safety in the workplace.
Common sleep disorders include insomnia, snoring and sleep apnoea (a breathing abnormality). Sleep problems can affect your health and safety, so if you’re feeling sleep deprived, see your doctor for advice.
Sleep occurs in five stages:
wake, N1, N2, N3, and REM. Stages N1 to N3 are considered non-rapid eye movement (NREM) sleep, with each stage a progressively deeper sleep. Approximately 75% of sleep is spent in the NREM stages, with the majority spent in the N2 stage.A typical night’s sleep consists of 4 to 5 sleep cycles, with the progression of sleep stages in the following order: N1, N2, N3, N2, REM. A complete sleep cycle takes roughly 90 to 110 minutes. The first REM period is short, and, as the night progresses, longer periods of REM and decreased time in deep sleep (NREM) occur.
Wake/Alert
EEG recording: beta waves – highest frequency, lowest amplitude (alpha waves are seen during quiet/relaxed wakefulness)
The first stage is the wake stage or stage W, which further depends on whether the eyes are open or closed. During eye-open wakefulness, beta waves predominate. As individuals become drowsy and close their eyes, alpha waves become the predominant pattern.
N1 (Stage 1) – Light Sleep (5%)
EEG recording: theta waves – low voltage
This is the lightest stage of sleep and begins when more than 50% of the alpha waves are replaced with low-amplitude mixed-frequency (LAMF) activity. Muscle tone is present in the skeletal muscle, and breathing tends to occur at a regular rate. This stage lasts around 1 to 5 minutes, consisting of 5% of total sleep time.
N2 (Stage 2) – Deeper Sleep (45%)
EEG recording: sleep spindles and K complexes
This stage represents deeper sleep as your heart rate and body temperate drop. It is characterized by the presence of sleep spindles, K-complexes, or both. Sleep spindles are brief, powerful bursts of neuronal firing in the superior temporal gyri, anterior cingulate, insular cortices, and thalamus, inducing calcium influx into cortical pyramidal cells. This mechanism is believed to be integral to synaptic plasticity. Numerous studies suggest that sleep spindles play an important role in memory consolidation, specifically procedural and declarative memory.
K-complexes are long delta waves that last for approximately one second and are known to be the longest and most distinct of all brain waves. K-complexes have been shown to function in maintaining sleep and memory consolidation. Stage 2 sleep lasts around 25 minutes in the first cycle and lengthens with each successive cycle, eventually consisting of about 45% of total sleep. This stage of sleep is when bruxism (teeth grinding) occurs.
N3 (Stage 3) – Deepest Non-REM Sleep (25%)
EEG recording: delta waves – lowest frequency, highest amplitude
N3 is also known as slow-wave sleep (SWS). This is considered the deepest stage of sleep and is characterized by signals with much lower frequencies and higher amplitudes, known as delta waves. This stage is the most difficult to awaken from, and, for some people, even loud noises (> 100 decibels) will not awaken them. As people age, they tend to spend less time in this slow, delta wave sleep and more time in stage N2 sleep. Although this stage has the greatest arousal threshold, if someone is awoken during this stage, they will have a transient phase of mental fogginess, known as sleep inertia. Cognitive testing shows that individuals awakened during this stage tend to have moderately impaired mental performance for 30 minutes to an hour.This is the stage when the body repairs and regrows tissues, builds bone and muscle and strengthens the immune system. This is also the stage when sleepwalking, night terrors, and bedwetting occurs.
REM (25%)
EEG recording: beta waves – similar to brain waves during wakefulness
REM is associated with dreaming and is not considered a restful sleep stage. While the EEG is similar to an awake individual, the skeletal muscles are atonic and without movement, except for the eyes and diaphragmatic breathing muscles, which remain active. However, the breathing rate becomes more erratic and irregular. This stage usually starts 90 minutes after you fall asleep, with each of your REM cycles getting longer throughout the night. The first period typically lasts 10 minutes, with the final one lasting up to an hour.REM is when dreaming, nightmares, and penile/clitoral tumescence occur.
Important characteristics of REM:
Associated with dreaming and irregular muscle movements as well as rapid movements of the eyes
A person is more difficult to arouse by sensory stimuli than during SWS
People tend to awaken spontaneously in the morning during an episode of REM sleep
Loss of motor tone, increased brain O2 use, increased and variable pulse and blood pressure
Increased levels of ACh
The brain is highly active throughout REM sleep, increasing brain metabolism by up to 20%
https://www.ncbi.nlm.nih.gov/books/NBK526132/
10 tips for healthy sleep
Try these tips to improve the quality of your sleep and to ensure you are properly rested the following day.
1. Stick to regular sleep patterns
Help your body establish a healthy sleep routine by going to bed and waking up at about the same time each day.
2. Spend the right amount of time in bed
While 8 hours of sleep is recommended for adults, some people require more sleep and some require less. Limit your time in bed to no more than 8.5 hours. If it takes you a long time to fall asleep, try going to bed later.
3. Use your bed for sleep, not screens
Computers, phones and TV can disrupt your sleep. Your mind needs to associate being in bed with sleeping rather than watching TV or using your computer. Don’t stay in bed if you’re wide awake.
4. Relax before bed
Establish a ‘buffer zone’ before bedtime where you’re not trying to solve any problems or are thinking about tomorrow. Find a relaxation technique that works for you instead. Avoid using your computer or phone during the ‘bedtime buffer zone’.
5. Be comfortable in your bedroom
Your room should be the right temperature, as well as quiet and dark. Make sure you have comfortable bedding and try to keep known stressors out of your bedroom.
6. Avoid alcohol, caffeine and cigarettes
While alcohol might make you sleepy initially, it can disrupt your sleep cycle, leading to poorer quality sleep. Caffeine and cigarettes are stimulants that can also stop you from sleeping well.
7. Don’t nap
Unfortunately, sleeping during the day can make it more difficult to fall asleep at night. If a nap is really necessary, limit the duration to half an hour and make sure you’re awake for at least 4 hours before going back to bed.
8. Don’t watch the clock
If you can’t sleep, checking the time can heighten your anxiety about not sleeping. If possible, take the clock out of your room.
9. Avoid sleeping pills
Sleeping pills, which can be addictive, don’t address the cause of your insomnia and won’t help you in the long term. Sleeping pills should only be prescribed by a trusted doctor who fully understands the reasons why you might be struggling to get good quality sleep.
10. Ask for help if you need it
If you regularly wake up feeling unrefreshed, are always restless in bed, have trouble getting to sleep or find that being tired is affecting your mood, see your doctor.
https://www.healthdirect.gov.au/sleep
Prior to the era of modern sleep research in the early 1920s, scientists regarded sleep as an inactive brain state. It was generally accepted that as night fell and sensory inputs from the environment diminished, so too did brain function. In essence, scientists thought that the brain simply shut down during sleep, only to restart again when morning came.
In 1929, an invention that enabled scientists to record brain activity challenged this way of thinking. From recordings known as electroencephalograms (EEGs), researchers could see that sleep was a dynamic behavior, one in which the brain was highly active at times, and not turned off at all. Over time, sleep studies using EEGs and other instruments that measured eye movements and muscle activity would reveal two main types of sleep. These were defined by characteristic electrical patterns in a sleeping person’s brain, as well as the presence or absence of eye movements.
The two main types of sleep are rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep. On an EEG, REM sleep, often called “active sleep,” is identifiable by its characteristic low-amplitude (small), high-frequency (fast) waves and alpha rhythm, as well as the eye movements for which it is named. Many sleep experts think that these eye movements are in some way related to dreams. Typically, when people are awakened from REM sleep, they report that they had been dreaming, often extremely vivid and sometimes bizarre dreams. In contrast, people report dreaming far less frequently when awakened from NREM sleep. Interestingly, during REM sleep muscles in the arms and legs are temporarily paralyzed. This is thought to be a neurological barrier that prevents us from “acting out” our dreams.
NREM sleep can be broken down into three distinct stages: N1, N2, and N3. In the progression from stage N1 to N3, brain waves become slower and more synchronized, and the eyes remain still. In stage N3, the deepest stage of NREM, EEGs reveal high-amplitude (large), low-frequency (slow) waves and spindles. This stage is referred to as “deep” or “slow-wave” sleep.
https://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem
Sleep was once considered an inactive, or passive, state in which both the body and the brain “turned off” to rest and recuperate from the day’s waking activities.
Scientists have since found that the brain goes through characteristic patterns of activity throughout each period of sleep, and that it is sometimes more active when we’re asleep than when we’re awake.
Understanding these patterns, and the factors that affect them, may help in making choices that will lead to better quality sleep.
Sleep patterns can be affected by many factors, including age, the amount of recent sleep or wakefulness, the time of the day or night relative to an individual’s internal clock, other behaviors prior to sleep such as exercise, stress, environmental conditions such as temperature and light, and various chemicals.
The internal mechanisms that regulate our almost ceaseless cycles of sleep and wakefulness make up a remarkable system. However, a variety of internal and external factors can dramatically influence the balance of this sleep-wake system.
The amount of sleep we obtain generally decreases and becomes more fragmented throughout our lifespan. These and other variations associated with age are covered at length in the essay Changes in Sleep with Age.
Other factors that affect sleep include stress and many medical conditions, especially those that cause chronic pain or other discomfort. External factors, such as what we eat and drink, the medications we take, and the environment in which we sleep can also greatly affect the quantity and quality of our sleep. In general, all of these factors tend to increase the number of awakenings and limit the depth of sleep.
Sleep timing
Newborns & infants (birth-1 year)
Sleep timing in newborns is distributed evenly across day and night for the first few weeks of life, with no regular rhythm or concentration of sleeping and waking. Newborns sleep approximately 16-18 hours per day discontinuously, with the longest continuous sleep episode typically lasting 2.5 to 4 hours. Newborns have three different types of sleep: quiet sleep (similar to NREM), active sleep (similar to REM), and indeterminate sleep. In contrast to children and adults, newborn sleep onset occurs through REM, not NREM, with each sleep episode consisting of only one or two cycles. These differences in sleep and sleep stages occur as circadian rhythms have not fully been determined.
Circadian rhythms begin to develop around two to three months of age, with greater durations of waking hours during the day and longer periods of sleep at night. At two months of age, the progression of nocturnal sleeping begins. Three months of age is when the cycling of melatonin and cortisol in a circadian rhythm occurs and when sleep onset begins with NREM. At this time, REM sleep decreases and shifts to the later part of the sleep cycle. The total NREM and REM sleep cycle is typically 50 minutes instead of the adult 90-minute cycle. At six months of age, the longest continuous sleep episode lengthens to six hours. At 12 months of age, infants typically sleep 14-15 hours per day, with most sleep now occurring in the evening with only one to two naps needed during the day.[6]
Toddlers (age 1 to 3) and children (age 3 to 9)
Around the ages of two to five, the total sleep time needed each day decreases by two hours, from 13 to 11 hours. By six years old, children will manifest circadian sleep phase preferences and tend toward being a night owl or an early riser. One study found that children appear to have longer REM sleep latencies than adolescents and thus spend more time in stage N3.[7]
Adolescents (age 10 to 18)
The total sleep time required for adolescents is 9-10 hours each night. Due to various pubertal and hormonal changes that accompany the onset of puberty, slow-wave-sleep and sleep latency time declines, and time in stage N2 increases. Around mid puberty, greater daytime sleepiness occurs than is seen at earlier stages of puberty.[8]
Adults (age 18+)
Adults tend to demonstrate earlier sleep time, wake time, and reduced sleep consolidation. Older adults (65+) have been shown to awaken approximately 1.5 hours earlier and sleep one hour earlier than younger adults (20 to 30).[9]
Gender Differences
Men tend to spend a greater time in stage N1 sleep and experience more nighttime awakenings, causing them to be more likely to complain of daytime sleepiness. Women maintain slow-wave sleep longer than men and tend to complain more often of difficulty falling asleep. Additionally, daytime sleepiness increases during pregnancy and the first few months postpartum.
