Sleep Cycles: Know Everything About the Schedules, Stages and Parts of the Body Involved in Sleep

It lasts about 90 minutes, and during that time we move through five stages of sleep.

Non-rapid eye movement sleep is made up of the first four stages of the sleep cycle, and it is the fifth stage of the sleep cycle when rapid eye movement sleep occurs.

In all four stages of non-rapid eye movement sleep the individual transitions from very light sleep during Stage 1 to very deep sleep in Stage 4.

In non-rapid eye movement sleep, there is little muscle activity and the eyes do not move normally. It is the deepest stage of sleep, but all muscles retain their ability to function as they normally do.

In rapid eye movement sleep of the sleep cycle, as the name implies, during this final stage of sleep we have bursts of rapid eye movement.

This is the stage of sleep in which most people’s dreams occur.

The eyes do not move constantly, but they do move from side to side, up and down.

These eye movements may be related to visual dream images, but that is unconfirmed, and in general, the reason for these eye movements that occur at this stage of the sleep cycle remain a mystery.

Although the eyes move rapidly, the muscles that move the body are paralyzed. Other important muscles, such as our heart and diaphragm (involuntary movements), continue to function as they normally do.

Typical dream

It turns out that it is not that simple to classify the stages of sleep, into stages of 4 to 6 90-minute sleep cycles as described above.

Over the course of the sleep journey, the amount of time spent in a particular stage of sleep begins to change.

During the first cycles of sleep, most of the time the sleep is deep, and it is a sleep of non-rapid eye movements.

During the later sleep cycles, much more time is spent in rapid eye movement sleep than in sleep accompanied by light sleep.

The complexity of sleep does not end there, apparently, the amount of non-rapid eye movement sleep and rapid eye movement sleep that is obtained is not only based on the environment where we sleep, but also depends on the time of day or of the night we do it.

Regardless of when they sleep, people tend to experience more non-rapid eye movement sleep in the early evening hours, from 11 p.m. to 3 a.m. and more rapid eye movement sleep in the late hours of the night, from 3 a.m. at 7 am.

So those people who sleep after these hours are getting more sleep from rapid eye movements in general than those who go to bed early.

As with many other aspects of the sleep cycle, the aspects of all this complexity in sleep cycles remain a mystery.

Changes in sleep throughout life

The typical adult sleeps approximately 8 hours per night, with much variability from one individual to another.

Adolescents tend to need 9 or more hours of sleep per night to be optimally alert the next day, and the need for sleep continues to increase as they move into early childhood.

It is not so simple to know how much sleep is needed at some point in life. Age also helps determine what type of sleep you have.

As you move from childhood to adulthood, you experience a reduction in the amount of deep sleep you get – non-rapid eye movement sleep of the sleep cycle.

This change occurs primarily in adolescence when about 40% of non-rapid eye movement sleep stages 3 and 4 are replaced by stage 2 sleep.

In addition to losing deep sleep, rapid eye movement sleep is also reduced as you age.

Newborns spend half of their total sleep time in rapid eye movement sleep, but at 2 years of age, that’s only a quarter of the time.

Babies also sleep in shorter cycles, only 50 to 60 minutes, and can fall asleep directly into rapid eye movement sleep.

A full cycle of sleep, estimated at 90 minutes, can help retain some ability to recover from fatigue, a 15-20 minute nap is ideal, and some research suggests that a nap of as little as 5 minutes may be beneficial for the organism.

Ideal time to sleep

Some researchers wonder if human bodies were designed for napping.

Some remote cultures are known to sleep in two phases: approximately six hours at night and an hour and a half nap in the afternoon.

This sleep schedule is similar to that adopted by Mediterranean cultures, well known for their mid-afternoon naps.

This type of sleep schedule may better fit the individual’s circadian rhythms, which tend to drop around 2pm.

Some companies are beginning to recognize the benefits of an afternoon nap, and companies have even sprung up in big cities to capitalize on this new nap craze.

The sleep cycle

There are five stages of sleep during the sleep cycle, scientists have used to classify the stages of sleep, the characteristics of the brain and the body of the individual during sleep.

Stages 1, 2, 3, and 4 are classified as non-rapid eye movement sleep and the fifth stage, rapid eye movement sleep.

In general, brain wave frequencies and amplitudes from an EEG are used to differentiate between different stages of sleep, along with other biological rhythms, including eye movements and muscle movements.

Non-rapid eye movement sleep of the sleep cycle

Stage 1 sleep

Stage 1 of the sleep cycle is the lightest stage of sleep, and in it the individual can be easily awakened. At this stage you may feel as if you are falling or have sudden muscle contractions (also known as ” hypnotic jerks “).

The brain rate of the EEG is slightly slower than during waking time. Muscle tone is present in skeletal muscles and breathing occurs at a regular rate.

Stage 2 sleep

Stage 2 which generally follows stage 1 and represents deeper sleep. During phase 2 of sleep, the individual is less able to be awakened.

Stage 2 sleep is also characterized by gnashing of teeth and sleep spindles.

During this second stage of the sleep cycle, your heart rate slows, your muscles contract and relax, and your body temperature drops as you prepare for a sound sleep.

Stage 3 and 4 of sleep or deep sleep

Stages 3 and 4 of the sleep cycle are progressively deeper stages of the sleep cycle.

These stages of sleep are also called “slow wave sleep” or delta sleep.

During slow-wave sleep, the EEG shows a much slower frequency with high-amplitude signals (delta waves).

A sleeper in “slow wave sleep” is often difficult to wake up. Some studies have shown that very loud noises, sometimes over 100 decibels, will not wake the individual during “slow wave sleep.”

As humans age, they spend less time in deep slow wave sleep and more time in phase 2 sleep.

Slow wave sleep is generally known as deep sleep, and comprises the deepest stage of non-rapid eye movement sleep.

In stage three we see the largest activation thresholds, such as difficulty awakening, among others.

After awakening, the person will generally feel quite groggy, and cognitive tests that have been administered after awakening from the third stage show that for about half an hour, and compared to the awakenings that occur in the other stages, mental performance is moderately impaired.

This is a phenomenon known as sleep inertia. When sleep deprivation occurs, there is usually a strong slow wave sleep rebound, suggesting that there is a need for slow wave sleep for the individual to have normal mental performance.

It now appears that slow wave sleep is a highly active state and not brain inactivity as previously believed.

In fact, brain imaging data shows that regional brain activity during non-rapid eye movement sleep is influenced by more recent waking experience.

Sleep cycle rapid eye movement dream

Stage 5 of the sleep cycle, or rapid eye movement sleep, is the physiologically very different stage of sleep from the other stages of sleep.

The EEG shows that it resembles the time to wake up. However, skeletal muscles are atonic, or motionless, and breathing is more erratic and irregular.

Various theories have been formulated where it is established that muscle atony evolved to protect the individual from injuries during sleep.

In this stage, the heart rate, breathing, and eye movement accelerate and the brain becomes more active, processing the things it has learned throughout the day to help it form memories and increase feel-good chemicals such as serotonin.

K Complexes and Sleep Spindles

Spindle activity is unique to non-rapid eye movement sleep, with most occurring at the beginning and end of non-rapid eye movement sleep.

The sleep spindles achieve brain activation in the superior temporal gyrus, the anterior cingulate, the insular cortices, and the thalamus.

Sleeping spindles are of different lengths, with slow spindles associated with increased activity in the area known as the superior frontal gyrus ranging between 11 and 13 Hz, and fast spindles associated with recruitment from both the hippocampus and the cortex. medial frontal and cortical regions of sensorimotor processing that vary between 13 and 15 Hz.

At this time it is unclear what is meant by these sleep spindles, but ongoing research is expected to reveal their role.

Defining K Complexes

K complexes are also unique to non-rapid eye movement sleep, and can be defined as single long delta waves lasting only one second.

Sleep spindles appear automatically during the early stages of sleep, usually stage two.

However, K Complexes can be induced at will by momentary noises, like someone knocking on a door.

Additional research on K complexes should be done because their function is currently unknown.

Dreaming during sleep of non-rapid eye movements

During the rapid eye movement phase of sleep, both dream intensity and dream memory enhancement have been reported to occur. This suggests that dreams normally occur in rapid eye movement sleep.

Dreams also occur during non-rapid eye movement sleep, however by comparison these dreams appear to be more mundane.

Dreams that occur during the non-rapid eye movement sleep stage of the sleep cycle generally occur after midnight, which is the period of time with the highest rate of non-rapid eye movement sleep.

This was discovered by a study in which subjects took naps for specific periods of time and were then forcibly awakened. It was observed that the dream was divided into:

  • Napping with only sleep dream of rapid eye movements.
  • Sleep-only naps from non-rapid eye movements using polysomnography.

The implication here is that the onset of rapid eye movement sleep is not necessary for dreaming, but rather that the actual processes that create rapid eye movement sleep produce alterations in a person’s sleep experience.

In the morning, and due to these changes, a subcortical activation occurs that is comparable to that which occurs during rapid eye movement sleep.

Therefore, during the morning hours, the subcortical activation that causes dreams occurs.

Non-rapid eye movement sleep muscle movements

The tonic drive to most upper airway respiratory muscles is suppressed during non-rapid eye movement sleep, with the following consequences:

  • Because intracellular calcium levels are decreasing, weakened muscle contractions are caused by rhythmic innervation, motor neurons hyperpolarize as tonic innervation is removed.
  • There is muscle weakness in the upper airway.
  • The diaphragm is typically actuated by the autonomous system and therefore typically does not have rapid eye movement sleep inhibition, and as such the suction pressure generated remains the same.
  • The upper airway narrows during sleep, increasing resistance and making the airflow in the upper airway noisy and turbulent.
  • A person is determined to be asleep by listening to their breathing, so the breath clearly becomes stronger once the individual falls asleep.
  • Not surprisingly, the increased tendency of the upper airway to collapse when breathing during sleep leads to snoring, which is simply the vibration of the upper airway tissues.
  • For overweight people, this problem is exacerbated when they sleep on their backs because excess fatty tissue can easily push and close the airway. This can even ultimately lead to sleep apnea.

Each stage of sleep plays a unique role in restoring the individual’s brain and body.

There are many studies of sleep deprivation that have shown that depriving subjects of specific stages of the sleep cycle has an adverse effect on brain and body functions.

Sleep cycles of a baby

Babies’ sleep cycles are unique compared to adult sleep cycles.

There are some differences, as well as similarities, between the way an adult and a baby sleep.


Newborns sleep 10 to 18 hours a day, more than any other age group.

The next best group of sleepers are three-month-old babies who sleep nine to twelve hours a day, plus naps.

Compared to adults who average less than seven hours of sleep per night.

Rapid eye movement sleep length

Adults go through sleep in various stages of rapid eye movement and non-rapid eye movement sleep divided into four stages.

Adults spend about 20 to 25 percent of the night in rapid eye movement sleep, and each sleep cycle takes about 90 to 120 minutes.

Meanwhile, in infants, the sleep cycle is more evenly divided between rapid eye movement sleep and non-rapid eye movement sleep.

Also, baby’s sleep cycles are shorter, lasting only about 50 minutes during the first nine months of life.

Brain development

In rapid or “active” eye movement sleep, a baby’s brain is developing, consolidating, and solidifying various cognitive and physical abilities.

Similarly, adults in rapid eye movement sleep also have active brains; It is at this stage that the mind is processing the events of the day, forming memories and releasing serotonin.

Waking night

While many adults sleep directly through the night, perhaps waking up once, because a baby’s sleep cycle is so short, they are prone to waking partially or fully during the transition from deep sleep to light sleep.

Most babies learn to calm themselves and send themselves back to sleep.

Babies are also more easily awakened when they first fall asleep during the active rapid eye movement stage of sleep.

During the “quiet” sleep stage of non-rapid eye movement sleep, babies are less likely to wake up (similar to how adults are less likely to wake up during deep sleep).

As the baby grows, the sleep cycles will start to look more and more like the adult version.

Less and less time is spent on rapid eye movement sleep while, at the same time, the sleep cycle lengthens.

Finally, by school age, the child will be sleeping in cycles of 90 to 100 minutes.