Stages of Sleep and Sleep Cycles

The typical sleep pattern has four stages: 1, 2, 3, and rapid eye movement (REM) sleep. These stages are usually cyclical and start with 1 then proceed through to REM before starting all over again.

A complete sleep cycle is typically 90-110 minutes with the stages about 5-15 minutes long each. The first sleep cycles typically have short REM sleeps and long deep sleep as the night progresses. Later in the night REM periods become longer as deep sleep shortens.


The Different Stages of Sleep?

There are four distinct sleep stages: REM sleep, and Non REM sleep that is divided into three – (Stages 1, 2, 3). Persons typically have short periods of wakefulness that may be intermittent or even occur before the different stages of sleep or happen when one is shifting sleeping positions.

Wake is the period when muscle tone and brain wave activity are at the most active.

Stage 1

NREM sleep happens in this stage which also happens to the lightest of all sleep stages. It is characterized by slow eye movement and is typically the stage where sleep can be easily disrupted by arousals and awakenings as one is normally just drowsy.

Brain activity slows down from wakefulness as the body relaxes from the reducing muscle tone. In rare cases, persons may experience abrupt muscle spasms, hypnic jerks and may even get a feeling of falling as they drift in and out of the stage.

Stage 2

Stage 2 of sleep is considered by experts as the first stage of NREM sleep. Arousals and awakenings are rare in Stage 1 with slow-moving eye rolls ceasing. The slowdown of waves continues with sleep spindles (intermittent bursts of rapid activity) intermingles with K complexes (sleep structures).

Both K complexes and sleep spindles are believed to work to protect the brain from involuntary awakening once the person sleeps. The heart rate slows down and the body temperature gradually goes down.

Stage 3

Deep NREM sleep happens in stage 3. The third stage combines slow and delta waves and is usually the most restorative sleep stage. Arousals and awakenings are rare in the stage and it may be difficult to awaken many people in this stage.

Parasomnias (night terrors, somniloquy, sleep talking, and sleepwalking happen during the deepest phase of stage 3.

REM Sleep

REM sleep alternatively referred to as rapid eye movement is known for the most part as the dreaming stage. Eye movements are side to side rapid and the brain waves get even more active as compared to stage 2 and 3. Arousals and awakenings are more frequent in REM. It is at this stage where being woken up leaves one feeling over sleepy and groggy.

During this stage a person can hold an intelligible conversation, they are responsive to external stimuli, and have their eyes open.

It is a transition between sleep and wakefulness. Most people will claim they were not asleep if they are awakened in this stage. Most of the light sleep happens in this stage that is also characterized by slow waves on the EEG, synaptic pruning, and memory consolidation.

The brain waves are similar to when one is awake and most dreams especially the most vivid will happen in the stage. The body is typically still during this stage.

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What Is a Sleep Cycle?

A sleep cycle refers to how one progresses through the different stages of NREM sleep through to REM sleep before it goes back to start in NREM. The normal person goes through a sleep cycle once every 90 – 120 minutes, which produces an average of four to five cycles a night.

However, no one goes from deep sleep to REM sleep but rather they proceed from non-REM sleep then to light sleep before ending up in deep sleep. They will then go from deep sleep to light sleep and finish off with REM before going back to light sleep.

Most people will typically have a sleep cycle that is characterized by short

Stage 1 phase where they experience slow rolling eye movements in a drowsy state combined with their bodies relaxing. While awakenings and arousals are common in this stage, it is still a critical stage as it sets the stage for the body to get into stage 2 – the first stage of the important NREM sleep.

Stage 2 is typically longer than stage 1 and for many people, it averages 40 – 60% of all the time spent sleeping.

Stage 3 in the sleep cycle is the next stage in the sleep cycle. It is a short sleep stage lasting an average of 5 – 15% of all time spent sleeping, though it is the most restorative. For adolescents and children, the stage usually takes a lot more of total sleep time.

REM can kick in at any time during the cycle though it will start at about 90 minutes after one starts sleeping. It is a short stage that forms the first REM of the sleep cycle. After REM the process goes right back to the beginning to go through stage 1, 2, and 3 combined with a return to REM, which progressively gets longer as one spends more time asleep.
For how long does the sleep cycle last? The first sleep cycle averages 90 minutes. Following that it rises to an average of 100 – 120 minutes. These combined cycles will typically take one through four or five cycles every night.


What Is Deep Sleep?

Stage 3 of NREM sleep is where deep sleep occurs. During this stage, the person experiences delta waves so called because they are brain waves with very large amplitude and slow speeds. While it is not that long it is the stage where the person will not be easily affected by external stimuli, which makes it the most restorative.

It can be difficult getting someone in deep sleep to wake up. After a person is deprived of sleep for a significant period of time, they tend to spend a lot of time in stage 3. Parasomnias such as bedwetting, night terrors, sleep talking and sleepwalking may occur. (Muscle activity makes people kick or talk in their sleep).

Deep sleep being the most restorative tends to reduce the drive for sleep. As such, one can still fall asleep during the night, even if they take a short nap in the afternoon. Nonetheless, taking a longer sleep in the day can make it difficult to fall into deep sleep since it reduces the sleep drive.

Human growth hormone which releases stress and restores the muscles and the entire body is usually produced during deep sleep. It also does restore the immune system. Nonetheless, there is much less information on deep sleep as compared to REM sleep. One unconfirmed effect of deep sleep is that it allows the brain to refresh so that it can take in new knowledge the following day.


When Does REM Sleep Occur?

A person’s sleep time is an average of 6 – 8 hours which sleep experts typically divide into two halves. For most people, the first half combines stage 2 and 3 and sporadic remission into short REM periods and stage 1 sleep. Over the course of the night stage 3 lasts less, stages 1 and 2 are relatively the same while REM become longer.

A person will typically have 3 – 5 REM periods during their sleep time with the longest of those coming right before they wake up for the day.

If they are woken before they get out of the REM period, they can get onto sleep inertia where they will feel heightened sleepiness that could be anything from several hours or just a few minutes.

People typically have shallow, irregular, and rapid breathing, temporary limb paralysis, and jerky eye movements in the REM period. The person will also experience higher brain wave activity similar to when they are awake.

Moreover, they will also experience an increase in blood pressure, heart rate, lose the ability to regulate body temperature and for males, they may get an erection.


In What Stage of Sleep Do Dreams Occur?

REM is an active sleep state where you are likely to have the most intense dreams. The rapid eye movements that happen during the stage are seen as rapid and sharp movements. Brain waves in REM sleep are mixed frequency and low amplitude waves that are higher than those common in stage 2 and 3.

A person will typically have an average of 4 -6 dreams every night. According to a French research study, everyone dreams even though some do not remember their dreams. However, a person will often remember their dreams if you happen to wake them up while they are in the REM sleep stage.

REM sleep is the stage where muscle paralysis occurs. The muscle paralysis also known as muscle atonia is a mechanism that prevents one from acting out their dreams. Persons with sleep apnea usually suffer the most during this stage as they lose muscle tone, particularly in the airways.

According to scientists, the sleep paralysis protects one from injury as they will not move limbs and body when acting out their dreams.

Respirations tend to become shallow and irregular, and heart rate and body temperatures become irregular during REM sleep


Brain Waves During Non-REM and REM Sleep

REM sleep was only discovered in 1953 when we finally had the machines that could effectively monitor brain wave activity. This just goes to show how young the field of sleep research is. Before the discovery, most scientists were of the opinion that a sleeping person did not have any brain activity.

Since 1953 scientists have debunked the idea that a lack of REM sleep could result in insanity though there is evidence that relieves symptoms of clinical depression. Recent research has linked memory and learning to REM sleep.

Brain Waves During the Sleep Cycle
Stage Frequency (Hz) Amplitude (micro Volts) Waveform type
Awake 15-50

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