What is the deepest level of sleep

Stages of Sleep - Neuroscience

The normal cycle of sleep and wakefulness implies that, at specific times, various neural systems are being activated while others are being turned off. A key to the neurobiology of sleep is therefore to understand the various stages of sleep. For centuries—indeed up until the 1950s—most people who thought about sleep considered it a unitary phenomenon whose physiology was essentially passive and whose purpose was simply restorative. In 1953, however, Nathaniel Kleitman and Eugene Aserinksy showed, by means of electroencephalographic (EEG) recordings from normal subjects, that sleep actually comprises different stages that occur in a characteristic sequence ( and ).

Figure 28.5

EEG recordings during the first hour of sleep. The waking state with the eyes open is characterized by high-frequency (15–60 Hz), low-amplitude activity (~30 μV) activity. This pattern is called beta activity. Descent into stage I non-REM (more...)

Figure 28.

Physiological changes in a male volunteer during the various sleep states in a typical 8-hour period of sleep (A). The duration of REM sleep increases from 10 minutes in the first cycle to up to 50 minutes in the final cycle; note that slow-wave (stage (more...)

Humans descend into sleep in stages that succeed each other over the first hour or so after retiring (). These characteristic stages are defined primarily by electroencephalographic criteria (Box C). Initially, during “drowsiness,” the frequency spectrum of the electroencephalogram (EEG) is shifted toward lower values and the amplitude of the cortical waves slightly increases. This drowsy period, called stage I sleep, eventually gives way to light or stage II sleep, which is characterized by a further decrease in the frequency of the EEG waves and an increase in their amplitude, together with intermittent high-frequency spike clusters called sleep spindles. Sleep spindles are periodic bursts of activity at about 10–12 Hz that generally last 1 or 2 seconds and arise as a result of interactions between thalamic and cortical neurons. In stage III sleep, which represents moderate to deep sleep, the number of spindles decreases, whereas the amplitude of low-frequency waves increases still more. In the deepest level of sleep, stage IV sleep, the predominant EEG activity consists of low frequency (1–4 Hz), high-amplitude fluctuations called delta waves, the characteristic slow waves for which this phase of sleep is named. The entire sequence from drowsiness to deep stage IV sleep usually takes about an hour.

Box C

Electroencephalography.

These four sleep stages are called non-rapid eye movement (non-REM) sleep, and its most prominent feature is the slow-wave (stage IV) sleep. It is most difficult to awaken people from slow-wave sleep; hence it is considered to be the deepest stage of sleep. Following a period of slow-wave sleep, however, EEG recordings show that the stages of sleep reverse to reach a quite different state called rapid eye movement, or REM, sleep. In REM sleep, the EEG recordings are remarkably similar to that of the awake state (see ). After about 10 minutes in REM sleep, the brain typically cycles back through the non-REM sleep stages. Slow-wave sleep usually occurs again in the second period of this continual cycling, but not during the rest of the night (see ). On average, four additional periods of REM sleep occur, each having longer durations.

In summary, the typical 8 hours of sleep experienced each night actually comprise several cycles that alternate between non-REM and REM sleep, the brain being quite active during much of this supposedly dormant, restful time. For reasons that are not clear, the amount of REM sleep each day decreases from about 8 hours at birth to 2 hours at 20 years to only about 45 minutes at 70 years of age.

Difference Between Deep Sleep and REM Sleep: Chase Dental Sleepcare: Dental Sleep Medicine

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All sleep is not the same. Deep sleep and REM, for instance, are two different forms of sleep which are often confused with one another. Each of these is a different stage of sleep, have specific characteristics. There are basically five stages of sleep. Deep sleep and REM are stages three and four of the sleep cycle.

REM Sleep

Rapid eye movement (REM) sleep is the deepest stage of sleep. As the name suggests, the irises of your eyes move rapidly during this stage. It is the fourth stage of sleep. This happens approximately 90 minutes after falling asleep. Your heartbeat is faster and irregular during this stage. Your body is largely inactive during this sleep.

REM sleep has intense brain activity. You experience vivid and active dreams. This sleep restores your brain and is good for your memory and learning. You can experience about three to five periods of REM sleep each night and each period lasts about 10 minutes, with the last one going up to an hour.

Stage 3: Deep Sleep

Deep sleep is often confused with REM sleep but there are differences between the two. This third stage of sleep is non-rapid eye movement sleep. Your body can enter this stage about half an hour or 45 minutes after falling asleep.

Unlike REM, deep sleep is associated with changes in the body rather than the brain. Your breathing is slow and heartbeat is regular. Your muscles are relaxed and you sleep through external noises. This form of sleep is very important as the body heals itself during this period – replaces cells, builds muscle tissue, and heals wounds. There are normally no dreams during this sleep. You feel disoriented when you wake up from this sleep. Deep sleep can last between 1-2 hours which is a quarter of your sleep time.

If you want to understand your sleep habits better, you consult one of our sleep specialists in the New York Metro area.

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STAGES OF SLEEP. DEEP, LIGHT AND REM SLEEP

We usually think of sleep as one single experience when we go to sleep every night. But our mind and body experience different phases of sleep during the night, each with its own characteristics and benefits.

DIFFERENT STAGES OF SLEEP

Sleep stages are formed by two different types of sleep: non-REM sleep (NREM sleep) and REM sleep (REM sleep). However, before we reach these periods of sleep, there is a stage where we are simply awake. To fall asleep, we begin this stage at rest.

NREM sleep stages is when our body slows down. All activity related to our brain waves, heartbeat, breathing and muscles is reduced to a lower level than when we are awake as our body uses this time to repair and improve our various systems. It is also a type of sleep that is rarely associated with the dream state (although vivid dreams can still occur, but are less likely).

Stages of non-REM sleep include:

First stage: the lightest stage of sleep. Our entry point is where we begin the transition into deeper sleep but are still easily woken up by the noises around us.
Second stage : Deeper stage of sleep when heart rate and body temperature decrease, but loud noises can still wake us up. This stage is characterized by rapid bursts of high-frequency brain waves thought to be important for learning and memory.
Third stage : Often referred to as deep sleep or non-REM sleep. It is very difficult for us to wake up in stage 3. In this stage, the body repairs tissues, builds bones and muscles, and strengthens the immune system.

REM sleep is REM sleep when our body experiences bursts of rapid eye movements and brain activity, similar to how our body behaves when we are awake. This is a type of sleep associated with vivid dreams and a kind of sleep paralysis where our muscles lose the ability to move (so it's not the stage where people sleepwalk, for example). REM sleep is vital to our memory and emotional regulation as our brain processes and clears what we no longer need.

HOW LONG IS A SLEEP CYCLE?

All of the above stages of sleep together form a whole sleep cycle, which usually lasts from 90 to 110 minutes. So, if you sleep eight hours a night, your body will complete several sleep cycles that will repeat until you wake up.

During the night, each sleep cycle is different: the deepest sleep occurs in the first half of the night, and the REM sleep occurs in the second half. So by the time you get to the early hours of the morning, you'll basically be alternating between NREM sleep and REM sleep.

HOW LONG IS EACH STAGE OF SLEEP?

During each sleep cycle, the body experiences each stage of sleep for varying durations, depending on how long you rested. Here is how long each sleep stage lasts in adults:

NREM Stage 1 : 1 to 7 minutes in our initial sleep cycle, which is 5-10% of total sleep duration.
NREM Stage 2 : 10 to 25 minutes in the initial sleep cycle and lengthens with each successive cycle, eventually accounting for 45 to 55% of our total night of sleep.
NREM Stage 3 : 20 to 40 minutes in our initial sleep cycle and accounts for about 15-25% of our total night's sleep.
REM sleep : only 1 to 5 minutes in the initial sleep cycle, but lengthens with each successive cycle.

HOW MUCH SLEEP SHOULD I GET IN THE RAPID PHASE?

As already mentioned, REM sleep begins in short periods and then gradually becomes longer. We usually have our longest REM sleep in the morning, just before waking up. That's why you often remember your dreams more clearly if you've had a long, uninterrupted night of sleep.

Since we have several intervals during the night, each of a different duration, it is not easy to determine exactly how much REM sleep we need. However, if we don't get enough sleep throughout the night, we are more likely to miss sleep cycles with longer sleep stages. Thus, an adult needs at least seven to nine hours of sleep each night.

DEEP SLEEP OR LIGHT SLEEP?

When we talk about light and deep sleep, we mean the stages of non-REM sleep. Light sleep occurs in stages 1 and 2, and deep sleep occurs in stage 3.

As we saw above, all stages of sleep are important because they work together in a cycle to allow our body to fully rest and regenerate throughout the night. Therefore, it is very important to get a good night's sleep in order to have a good rest and benefit from the different stages.

Often, when our sleep is disturbed, we lose deep sleep. This means we miss out on the time our bodies need each night to reap the benefits of phase 3 sleep, such as improving your memory and learning functions, supporting cell growth, and boosting your immune system.

If you are interested in getting enough deep sleep every night, you should rethink your sleep patterns to create the ideal conditions for a full night's rest. Some studies have also shown that exercising 90 minutes before bed can increase the duration of deep sleep (but may decrease the duration of REM sleep).

While deep sleep is vital, light sleep also has its benefits. For example, when we wake up naturally during this part of our sleep cycle, we are more likely to feel rested and ready to start the day—a very different morning feeling than when an alarm goes off while you are in deep sleep.

SLEEP CHANGES WITH AGE

If you slept until noon as a teenager and wake up at dawn as an adult, you have probably noticed how your sleep changes throughout your life. This is how our need for sleep and experience of sleep change as we age.

Newborns and infants : Babies initially need 14 to 17 hours of sleep, usually in chunks throughout the day. By their first birthday, they typically sleep between 11 and 14 hours a night, mostly at night, with little to no daytime naps. Interestingly, newborns have a completely different sleep architecture, and it also takes their bodies three months to understand circadian rhythms.
Young children : although the need for sleep (including naps) is decreasing, children around the age of five still need about 12 hours per day. As children get older through their teens, the need for sleep decreases to about 9-11 hours each night.
Adolescents : Adolescents need about 8-10 hours of sleep each night and may experience afternoon sleepiness at various stages of puberty.
Adults : Sleep cycles remain relatively stable throughout adulthood until about age 65, with the average adult needing 7-9 hours of sleep each night.
People over 65 years of age : After this age, people have slightly less NREM sleep and spend most of their sleep cycle in stage 2. This means that older people do not sleep as soundly as they did when they were younger and often need a little less sleep (about 7-8 hours).

TRACKING SLEEP STAGES

One way to understand your sleep cycles is to track them. Nightly Recharge™ by Polar gives you a detailed analysis of your sleep every morning so you can evaluate the amount and quality of your sleep.

Nightly Recharge™ has two separate metrics: Sleep Charge and ANS Charge. Sleep Charge is the charge of sleep, it is associated with the phases of your sleep. Your sleep score shows sleep duration, sleep continuity, duration of any breaks, and details what percentage of your rest was spent in REM, deep, and light sleep. Sleep Charge then compares your sleep score to your 28-day average to show how much you recovered overnight.

ANS Charge means heart rate, heart rate variability and breathing during sleep. This indicator is needed to understand how well your autonomic nervous system relaxes during the first four hours of sleep each night. Nightly Recharge™ then uses these two metrics to evaluate your overall recovery so you can decide how active you need to be during the day.

The science of sleep: research and advice

July 31, 2014 Health

Many of us give up healthy sleep for work or play. In the meantime, research proves that lack of sleep cannot be filled with anything else. Sleep is essential for health, productivity, and even youthfulness.

Iya Zorina

Author of Lifehacker, athlete, CCM

Scientists have paid attention to sleep relatively recently, which is strange if you remember how much of your life we spend in a dream. After the scientific interest in sleep processes arose, so-called sleep centers appeared at Harvard and the University of Pennsylvania, numerous studies were carried out and conclusions were drawn. In this article, you'll learn what the science of sleep is, why many people can't fall asleep, and some practical exercises for healthy sleep and more energy.

The first steps in the science of sleep

The pioneer of chronobiology was the French scientist Michel Siffre, who studied biological rhythms in a harsh experiment on himself. He lived in an underground cave with a bed, a table, a chair, and a phone to call his research team.

Michel Siffre during experiment

His underground home was lit with just one light bulb with a soft glow. From food - frozen foods, several liters of water. There were no clocks, no calendars, and no way of knowing what time it was on the surface, day or night. And so he lived alone for several months.

A few days after the descent into the cave, Siffre's biological clock began to work. Later he recalled how he felt during the experiment:

My sleep was wonderful. My body itself chose when to sleep and when to eat. It is very important. My sleep-wake cycle did not last 24 hours, like people on the surface of the earth, but a little longer - about 24 hours and 30 minutes.

So despite the absence of sunlight and any knowledge of whether it was day or night, his circadian rhythms continued to work.

After this experiment, many scientists became interested in the study of sleep. New research has helped to figure out how much sleep you need, why you need to do it and how you can make up for the lack of sleep.

How much sleep do you need

How much sleep do you really need? To answer this question, let's turn to the experiment of scientists from the University of Pennsylvania and the University of Washington.

The researchers recruited 48 healthy men and women who were accustomed to sleeping 7-8 hours a night. The participants were then divided into four groups.

People from the first group had to go without sleep for three days, from the second - to sleep 4 hours a day. Participants in the third group were allowed to sleep 6 hours a day, and those in the fourth group were allowed to sleep 8 hours a day.

Three groups who slept 4, 6, and 8 hours a day had to do this for two weeks. During the experiment, scientists observed the physical health and behavior of the participants.

As a result, the group of participants who slept 8 hours a day did not experience any impairment during the entire experiment - cognitive decline, deterioration in reaction or memory lapses. At the same time, in people who slept 6 and 4 hours a day, all indicators gradually deteriorated.

The 4-hour group performed worse, although not by much, than the 6-hour group. In general, two significant conclusions were drawn from the experiment.

First, lack of sleep tends to accumulate. In other words, sleep deprivation has a neurobiological cost that only increases over time.

After one week of the experiment, 25% of the participants who slept 6 hours a day fell asleep intermittently at different times during the day. After two weeks, people in this group had the same performance as if they had spent two days without sleep at all.

Lack of sleep gradually builds up.

The second conclusion is no less important: the participants did not notice a decrease in their performance. The participants themselves believed that their performance deteriorated over several days, and then remained at the same level. In fact, their performance continued to decline throughout the experiment.

We do not notice a decrease in cognitive functions with a lack of sleep.

It turns out that we assess our condition very poorly and cannot accurately determine how well our cognitive functions work. Especially in today's environment of constant social activity, caffeine and many other factors that help to feel fresh and alert, even if in fact it is far from being the case.

The cost of lack of sleep

The irony is that many of us suffer from lack of sleep in order to earn more money. But no matter how many extra hours you spend working instead of getting enough sleep, it will not increase your productivity much. Your attention, memory and other functions deteriorate, and you do all tasks more slowly and worse.

Studies have shown that reduced work efficiency due to lack of sleep costs US businesses a huge amount. An average of $100 billion is lost per year.

Here's what George Belenki, director of the Center for Research on Sleep and Performance at the University of Washington, had to say about it:

If your job is mental, you're paying productivity for lack of sleep.

After that, a completely logical question arises: how much time do you need to sleep in order not to accumulate fatigue and reduce productivity?

Based on research data, we can say that this time is from 7 to 7. 5 hours. In general, the experts agreed that 95% of adults need 7 to 9 hours of sleep per night to perform well.

Most adults are better off getting 8 hours of sleep per night, and children, teens and the elderly are even better off.

How sleep works: the sleep and wake cycles

The quality of your sleep is determined by a process called the sleep and wake cycle.

There are two important moments in this cycle:

Non-REM sleep (also known as deep sleep).
REM sleep (REM, REM phase).

During slow wave sleep, the body relaxes, breathing becomes calmer, blood pressure drops, the brain becomes less sensitive to external stimuli, which makes it difficult to wake up.

This phase is of great importance for the renewal and restoration of the organism. During slow-wave sleep, growth hormones are produced in the pineal gland, which provide tissue growth and muscle repair.

Researchers also suggest that the immune system is restored during non-REM sleep. So the slow phase of sleep is especially important if you play sports. Some professional athletes, like Roger Federer or LeBron James, slept 11-12 hours a night.

Another example of how sleep affects physical performance is a study conducted on Stanford University basketball players. During the study, the players slept at least 10 hours per night (as opposed to the 8 hours they were used to).

The experiment lasted five weeks, during which the researchers assessed the speed and accuracy of the players in comparison with their usual results.

It turned out that just two extra hours of sleep increased the number of successful throws by 9% and reduced the time to sprint 80 meters by 0.6 seconds. So, if you have heavy physical activity, the slow phase of sleep will help you recover.

REM sleep is just as important for the mind as slow sleep is for the body. Most of the time when you sleep, the brain is calm, but when the REM phase comes, it becomes active. This is the phase during which you dream and your brain redistributes information.

During the REM phase, the brain erases unnecessary information and improves memory by linking the experiences of the last 24 hours with previous experiences, facilitating learning and provoking the growth of neural connections.

At this time, the body temperature rises, blood pressure rises, and the heart beats faster. In addition to this, the body is moving. In general, REM sleep occurs three to five times a night for a short period of time.

A person cannot function normally without both phases of sleep. Sleep deprivation affects health: immunity goes down, consciousness becomes “foggy”, the risk of infectious diseases increases, blood pressure and the risk of heart disease increase. In addition, sleep deprivation threatens mental illness and reduces life expectancy.

The slow phase of sleep helps to restore physical health, the fast phase - mental capabilities.

However, despite the great importance of sleep for the body, the quality and duration of sleep change throughout life.

Age-related changes in sleep

Based on research from Harvard Medical School, it can be said that as people age, it becomes increasingly difficult to fall asleep. This phenomenon is called sleep delay. And sleep efficiency — the percentage of time you spend in bed while you sleep — is also declining.

On average, 80-year-olds have 62% less long sleep than 20-year-olds. There are many factors that affect the aging of tissues, and if this phase of non-REM sleep is shortened, the aging process is even faster.

Healthy sleep is your best weapon against rapid aging.

How to recover from a lack of sleep

Most adults need 8 hours of sleep to keep their performance at their best. Since older people have trouble sleeping, they can make up for the lack of nighttime sleep by taking a nap during the day.

In any case, if you understand that you need to take a nap, it is better to do it once in the middle of the day than to periodically fall asleep during the day and evening.

In general, the body recovers well after short-term lack of sleep. For example, if you had a rough night where you managed to get 2-4 hours of sleep, the next night 9-10 hours of sleep will fully restore your body.

It's just that your body will spend more time in REM and NREM sleep to make up for the lack of sleep last night.

There is no need to plan how much time your body will spend in REM and non-REM sleep. It knows better how much sleep and how much sleep is needed for recovery, so you will not be able to control this process.

And remember that nothing can replace sleep. If you are bound to stay awake longer today, make sure you sleep longer than usual the next night.

Circadian rhythms

How are your sleep and wake cycles organized?

With the help of circadian rhythms. These are biological cycles of different processes that occur within 24 hours.

Here are a few key moments of the 24-hour cycle:

6:00 - cortisol levels rise to wake your body;

7:00 - melatonin production stops;

9:00 am - Sex hormone production peak;

10:00 - peak of mental activity;

14:30 - the best level of coordination of movements;

15:30 - best reaction time;

17:00 - better work of the cardiovascular system and muscle elasticity;

19:00 - the highest level of blood pressure and the highest body temperature;

21:00 - melatonin begins to be produced to prepare the body for sleep;

22:00 - the work of the digestive system calms down, as the body prepares for sleep;

2:00 - deepest sleep;

4:00 - lowest body temperature.

Of course, these are only approximate rhythms, since they are individual for each person and depend not only on daylight, but also on habits and other factors.

In general, circadian rhythms are influenced by three main factors: light, time, and melatonin.

Light

Light is one of the most important factors that sets the circadian rhythm. Staying in bright light for about 30 minutes can reset your rhythms, no matter what time it is.

In general, when the sun rises and light enters your closed eyes, it signals the beginning of a new cycle.

Time

The time of day, your daily schedule, and the order in which you are used to performing different tasks all affect your sleep and wake cycles.

Melatonin

This is a hormone that causes drowsiness and controls body temperature. Melatonin production depends on a daily, predictable rhythm. Its amount increases at night and decreases when it becomes light.

How to sleep better

Here are some rules for falling asleep quickly and sound sleep.

Avoid caffeine

If you have trouble sleeping, it's best to eliminate caffeine from your diet altogether. But if you can't turn on in the morning without a cup of coffee, at least don't drink it after dinner.

Quit smoking

Many people who quit or have quit smoking have experienced that cigarettes have a bad effect on sleep. After you quit smoking, it will be easier to fall asleep, the number of awakenings at night will decrease.

Only use the bedroom for sleep and sex

Remove the TV from the bedroom, do not bring a laptop or tablet. The ideal sleeping environment is a dark, cool, and quiet bedroom, so try to make it that way.

Exercise

Physical activity helps the body and brain to turn off at night. This is especially true for older people. It has been proven that mobile, active older people sleep much better. However, at least three hours should pass between classes and sleep, so that the brain and body have time to calm down and prepare for sleep.

Temperature

Most people sleep better in a cool room. The ideal temperature in the bedroom is 18-21°C.

Sounds

Quiet room ideal for a good night's sleep. But if you find it difficult to fall asleep in complete silence, you can turn on white noise.

No alcohol

A small (or very large) amount of alcohol can help you sleep, but the quality of this sleep is poor. During such sleep, the REM phase is reduced, so you do not have a good rest, even if you slept all night.

How to get ready for bed

Here's what to do to avoid insomnia.

Set a daily schedule

Our bodies love systems. In essence, the circadian rhythm is your daily routine on a biological level. Go to sleep and wake up at the same time every day.

Get in the habit of turning off all electronics an hour or two before bed. Light from a computer, TV or smartphone delays the production of melatonin, which helps the body prepare for sleep.

In addition, working before bed increases brain activity and can increase stress levels, which can affect sleep. Instead of checking your work email, read a paper book. This is a great way to get away from the screen and learn something interesting and useful.

Use relaxation techniques

Researchers say that 50% of cases of insomnia are caused by intense emotional distress and stress. Find a way to reduce stress and you'll find it much easier to fall asleep.

Proven methods include journaling, breathing exercises, meditation, exercise.

Don't miss a nap

Afternoon nap helps replenish sleep cycles. This is especially useful for those who cannot sleep well at night.

How to be more energetic in the morning

Drink a glass of water in the morning

Your body has gone 6 to 8 hours without water. Feeling sleepy in the morning (of course, if you slept enough) can be caused by a lack of moisture. So a glass of cool water may well refresh you.

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