Index
It lasts about 90 minutes, and we move through five stages of sleep during that time.
Non-rapid eye movement sleep comprises 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 non-rapid eye movement sleep stages, the individual transitions from very light sleep during Stage 1 to profound sleep in Stage 4.
There is little muscle activity in non-rapid eye movement sleep, and the eyes do not move normally. It is the deepest stage of sleep, but all muscles retain their ability to function as they usually do.
In rapid eye movement, the sleep of the sleep cycle, as the name implies, we have bursts of rapid eye movement during this final stage of sleep.
This is the stage of sleep in which most people’s dreams occur.
The eyes do not move constantly, but they move from side to side, up and down.
These eye movements may be related to visual dream images, but that is unconfirmed. In general, the reason for these eye movements that occur at this stage of the sleep cycle remains a mystery.
Although the eyes rush, the muscles that move the body are paralyzed. Other essential muscles, such as our heart and diaphragm (involuntary movements), continue to function as they usually do.
Typical dream
It is not that simple to classify the stages of sleep into stages of 4 to 6 90-minute sleep cycles as described above.
Throughout 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 rest is deep, and it is asleep with non-rapid eye movements.
Much more time is spent in rapid eye movement sleep during the last sleep than in rest accompanied by light sleep.
The complexity of sleep does not end there. The amount of non-rapid eye movement sleep and rapid eye movement sleep obtained is based not only on the environment where we sleep but also on the time of day or 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 pm to 3 am, and more rapid eye movement sleep in the late-night hours, from 3 am to 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 elements 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 nine 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 two 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 complete cycle of sleep, estimated at 90 minutes, can help retain some ability to recover from fatigue. A 15-20 minute nap is ideal. Some research suggests that a rest of as little as 5 minutes may benefit the organism.
The 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 sleep schedule may better fit the individual’s circadian rhythms, which tend to drop around 2 pm.
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 them to classify the stages of sleep, the characteristics of the brain, and the individual’s body 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 regularly.
Stage 2 sleep
Stage 2 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 the 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.
The EEG shows a much slower frequency with high-amplitude signals (delta waves) during slow-wave sleep.
A sleeper in “slow-wave sleep” is often difficult to wake up. Some studies have shown that sometimes over 100 decibels, loud noises will not wake the individual during “slow-wave sleep.”
As humans age, they spend less time 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 most significant activation thresholds, such as difficulty awakening, among others.
After awakening, the person will generally feel quite dizzy, 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 locations, mental performance is moderately impaired.
This is a phenomenon known as sleep inertia. There is usually a solid slow-wave sleep rebound when sleep deprivation occurs, suggesting a need for slow-wave sleep for the individual to have average mental performance.
It now appears that slow-wave sleep is a highly active state and not brain inactivity, as previously believed.
Brain imaging data shows that recent waking experiences influence regional brain activity during non-rapid eye movement sleep.
Sleep cycle, rapid eye movement dream
Stage 5 of the sleep cycle, or rapid eye movement sleep, is the physiologically very different rest stage 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.
The heart rate, breathing, and eye movement accelerate in this stage. 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 related to recruitment from both the hippocampus and the cortex. Medial frontal and cortical regions of sensorimotor processing vary between 13 and 15 Hz.
At this time, it is unclear what these sleep spindles mean, 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 unknown.
Dreaming during sleep of non-rapid eye movements
During the rapid eye movement phase of sleep, dream intensity and dream memory enhancement have been reported. This suggests that dreams typically 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 happen after midnight, the period with the highest rate of non-rapid eye movement sleep.
This was discovered by a study in which subjects took naps for specific periods 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 instead 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 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, generally 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 becomes more robust 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 quickly 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.
Many studies of sleep deprivation have shown that depriving subjects of specific sleep cycle stages hurts brain and body functions.
Sleep cycles of a baby
Babies’ sleep cycles are unique compared to adult sleep cycles.
There are some differences and similarities between the way an adult and a baby sleep.
Duration
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 sleep in various stages of rapid eye movement, and non-rapid eye movement sleep is divided into four phases.
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, a 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 develops, consolidates, and solidifies 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 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.
