Stages of Sleep

Before the 1950s. The scientists mostly believed that sleep is just a simple shutdown of the body and mind necessary for recharging for the next day, and this ‘shutdown’ was a kind of a passive mode.  A lot of research has been done on sleep since then, and now the scientists know well the sleep n not such a simple phenomenon and that it has various stages. Moreover, it is not as passive as they thought earlier. It is quite active in certain ways. They now know that while the person is asleep, the brain is busy performing a range of activities. The sleep has two main cycles, which are known as the NREM or REM sleep 1. REM stands for Rapid Eye Movement.  There are four distinct stages of sleep, and three of these are a part of the NREM sleep while the fourth one is the REM sleep.  Some analysts divide the stage into two and thus consider REM to be the fifth stage.

Cyclic Nature of Sleep

While sleeping, the human body goes through these 4v stages of sleep, and these include both NREM and REM stages. Normally, the body cycles through these stages not once but at least 4 to 6 times.  The average number of minutes in each stage is 90 minutes. As the night progresses and the individuals continue to sleep, the number of NREM stages becomes fewer, and the duration of the REM sleep episodes start increasing 2. Circadian rhythm seems to regulate sleep cycles. Circadian rhythm is a 23-hour internal clock which runs in the background of the human brain and cycles between the stages of alertness and sleepiness at regular intervals. It is also popularly known as the sleep/wake cycle. A portion of the brain controls the circadian rhythm in the human body 3. The circadian rhythm, however, changes throughout human life. It is very different in new-borns, children, teens, adults, and the elderly. New-borns normally spend half of the time of their total sleep hours in the REM stage of the sleep. They are, in fact, able to enter the REM sleep directly. New-borns also exhibit the characteristic of sleeping in short intervals and total during 24 hours; they obtain anywhere from 12 to 18 hours of sleep.  As the child reaches the age of 5 to 10 years, the demand for sleep has decreased to 10 hours.  By the time the child turns into a teen; the sleep requirement is further brought down to only 8-9 hours. As the teen grows further and becomes an adult, usually 7 to 8 hours of sleep is quite sufficient. The circadian rhythm also controls the release of certain hormones in the body, which are essential for the functioning of the body. These include melatonin, norepinephrine, adrenocorticotropic hormone (ACTH), and prolactin 4.

The Wake Stage; Stage W

This is also known as Stage W. It depends a lot on whether the individual’s eyes are open or closed. When the person’s eyes are open, this type of wakefulness means that both alpha and beta waves which are produced in the brain are present. However, beta waves are more dominant. As the person starts to get drowsy, the eyes start to close, and the predominant rhythm is now the alpha rhythm.  In case, the alpha waves are more than 50% and are dominating; the person is said to have entered the first NREM sleep stage called Stage W or the wake stage 5.

Stage 1, NREM Sleep

The first stage is the lightest stage of sleep. It starts as soon as more than half of the alpha waves that are present are now replaced by low-amplitude mixed-frequency (LAMF) activity. The breathing continues to occur normally at the regular rate, and there is still muscle tone present in the skeletal muscle.  Normally, this stage would last only for anywhere from 1 to 5 minute, and it consists of about 5% of the total sleep cycle 6. According to some experts, it could last for as long as 7 minutes.  This stage signifies that the individualism in a very light stage of sleep and is, in fact, a bit alert as well. The person can be very easily woken up during this stage. Often people are known to indulge in ‘catnaps’ during this sleep stage 1.

Stage 2 NREM Sleep

This sleep stage is representative of still light sleep but its deeper than that of stage 1. During this stage, the body temperature and the heart rate starts to drop. The brain starts to produce sleep spindles, which are sudden increases in brain wave frequency. K-Complexes start to appear as well or sometimes both might be present. A K-Complex is detected as electroencephalography (EEG) waveform. In a healthy human, the K-Complex represent the largest event. These are more frequent in this second stage of the sleep cycle 7.

The speed spindles start to activate the anterior cingulate, superior temporal gyri, thalamus, and the insular cortices. The K-complexes are representative of the fact that the person is now transitioning into a deeper sleep.  These are single, long delta waves which can last no more than a second. The onset of this deeper stage of sleep is an indication that the person is now starting to enter the third stage of NREM sleep.  Gradually, all the waves would get replaced by the delta waves. The total duration of stage 2 of NREM sleep is usually 25 minutes in the first cycle. However, it starts to increase in the subsequent cycles. Ultimately, it consists of half of the total sleep of the individual 6. Some people who would like to schedule a powernap do it in this stage. They want to wake up after this stage of sleep 1.

Stage 3 NREM

This is indeed the deepest stage of the sleep. During this stage, the delta waves, which are high amplitude signals have a much lower frequency. It is very hard to awaken from this stage of sleep as it is very deep.  Many people would not wake up during this stage of sleep, even if a noise of 100 dB is produced around them. With the increase in the age of an individual, the time spent during this slow delta wave stage of sleep starts to decrease. Instead, they start spending more time in the Stage 2 NREM sleep. It is important to note that during this stage of sleep, the arousing threshold is quite high. This means that if someone is forced to awaken during this stage of sleep, he/she would have a transient stage of mental fogginess. In other words, this phenomenon is called ‘sleep inertia,’  Sleep inertia could continue to last from 1 to 4 minutes. Many other tests have been conducted, and it has been shown that for at least 30 minutes to 1 hour, a person who is awakened from this stage of sleep would tend to have his/her mental performance temporarily and moderately impaired. In fact, during this stage, the body is busy in regenerating and repairing its tissues. It also builds muscles, bones, and give further strength to the immune system 6. For this reason, it is not a good idea to force anyone to awaken during this stage of sleep as it disturbs the regeneration and repairing of the body’s muscle, bones, and the immune system.

REM Sleep

The sleeper normally enters this stage of sleep around 90 minutes after initially falling to sleep. Every REM stage could last up to 60 minutes. Normally, an average adult would easily have 5 or 6 REM cycles every night. AS the person enters this stage of sleep, the brain starts to become more active. People tend to dream during this very stage of sleep.  The reason this stage of sleep has been named as such, i.e. Rapid Eye Movement or REM is that the eyes start to jerk quickly in different directions.  The blood pressure, as well as the heart rate of the individual start to increase, and the breathing, also starts to become faster, shallow, and irregular 1. The EEG in this stage of sleep would be quite similar to a person who is fully awake except the skeletal muscles would be without movement and would be atonic. The diaphragmatic breathing and eye muscles are, however, an exception. These remain active during this stage, as described above. The first REM sleep might last only 10 minutes, but each successive REM stage would be longer until the last one could reach a duration of up to 60 minutes 6.

Evaluation of Sleep

The evaluation of sleep is critically performed through a piece of equipment known as Polysomnogram. This instrument measures brain waves or EEG, chin, and leg muscle movements, eye movement, nasal airflow and pressure, pulse oximetry and chest and thorax movement. All of these studies are integrated and are used to examine and investigate each sleep stage for movement as well as breathing during the individual’s sleep. The tests are carried out overnight, and the individual needs to be monitored for at least 6 hours 6. Sleep studies are carried out through the use of Polysomnography to identify the sleep patterns and to examine if these are disrupted or not, and if they are, the reasons are investigated. Doctors usually recommend such sleep studies in case an individual ha asleep related breathing disorder. For such individuals, breathing suddenly stops and starts during their sleep. These studies are also conducted if the individual involuntarily starts to extend or flex his/her legs while sleeping.  This is normally the case with individuals who have got the restless leg syndrome. In certain cases, if the individual is suspected of having Narcolepsy, the tests are conducted as well.  The person starts to experience drowsiness during the day time, and it becomes quite overwhelming. He/she could also face attacks if sleep during this drowsy condition. Certain individuals have what is known as a REM sleep disorder.  It involves them to act out their dreams while they are sleeping.

Similarly, any other unusual behavior during sleep necessitates such a study as well. For any person who is going through such unusual behavior or activities during sleep, the doctors would almost always conduct these types of studies. The person could be sleepwalking, or he/she could be moving around a bit too much during sleep or carrying out rhythmic movements. For some people who have insomnia but it cannot be explained with any reason, they need to go through this type of testing as well 8.


Many individuals experience sleep disorders. These people could suffer from sleep fragmentation after that the sleep comes in fragments and is not continuous.  As these people tend to fall into the later stages, i.e. deeper stages of their sleep, the upper airway of their breathing mechanism starts to collapse, and it starts to cause interference in their normal breathing pattern. This results in the body shifting backward in the sleep cycle to the lighter and earlier stages of sleep where it would be able to breath normally.  However, as described above, the deep stages of the sleep are extremely important for the growth and regeneration of tissue and the strengthening of the bone, the muscle, and even the immune system. Lack of deep sleep leads to disruptions in this mechanism — people who have this sleep apnea.

Sleep Apnea

These people who have these issues referred to as sleep apnea cannot go through the normal stages of sleep.  The reduction in the NREM Stage 3 as well as in the REM sleep is imminent in these individuals as the breathing mechanism cannot afford to stay in these stages due to the fear of breathing collapse and keeps reverting to the earlier lighter stages of sleep. However, in those stages, the immune system and bone and muscle strengthening, as well as tissue growth and generation, do not take place.  This causes these people to not only become drowsy during the day time but also, they tend to get other health-related issues in the long term. Sleep apnea is of 2 types. These are the central sleep apnea and the obstructive sleep apnea. Central sleep apnea sets in when the brain is unable to signal the respiratory mechanism while sleeping properly. The obstructive sleep apnea, on the other hand, relates to the mechanism related issue in which the upper airways are either partially or completely blocked 9.

REM Stage-related Disorders

When an individual is going through the REM Stage of the sleep, he/she normally does not move as the muscles are paralyzed.  This temporary paralysis is called Atonia.  If this paralysis is disturbed for any reason, the person can start to act out his/her dreams. This is termed as Rapid Eye Movement (REM) sleep disorder.  In the presence o0f such an abnormality, the normal Atonia would not be obtained, and abnormal movements would start to occur in episodes throughout the night. These are all related to the REM sleep stage because this is the stage in which the person dreams.  The scientists are not yet fully cognizant of the reasons and causes of this disorder; however; it can be associated with a certain type of degenerative neurological conditions. These include Lewy body dementia or Parkinson’s disease 10.


This is sleepwalking. It is, in fact, a common occurrence in many children who are in their school-going age. These children make movements that seem to have a purpose. It must be understood that this is not the process of acting out their dreams. Dreams are possible during the REM stage of sleep, and the body is completely paralyzed during this stage.  Sleepwalking, therefore, does not occur in this stage. It occurs at a time when the sleep cycle has not matured yet, and the proper sleep/wake cycles have not yet become regulated.  Sleepwalking is linked to other very common behaviors, like eating, dressing, urinating, etc. Therefore, it is important to note that sleepwalking happens during non-REM related stages. It mostly occurs during the NREM sleep stage 3 11.

The sleep stages have a lot of clinical consequences as well as far as the understanding of sleep quality as well as sleep disorders, is concerned. Sleep studies are important for this purpose and give the experts deep insights into the causes of sleep disorders related to the various stages of sleep as described above. Sleep experts and clinic related doctors often treat sleep disorders based on the information they obtain in these studies conducted over individuals with sleep disorder related issues. The diagnosis is based on which sleep stage is affected by the disorder, and the remedies are prescribed accordingly. Even drugs prescribed tend to decrease the stage of sleep that has the disorder to mitigate the effects of the disorder. Drugs that treat insomnia target Sleep stage NREM 3 and REM to increase arousal threshold. Thus, the understanding of the sleep stages is paramount tithe treatment of many types of sleep disorders. Each study conducted upon individuals having sleep disorders examines each sleep stage separately and then, based upon the results obtained analyses, the data collected to pinpoint the problem to a particular sleep stage. This helps the experts to diagnose the right remedy for the individual. The treatment directly targets the stage of sleep that is being affected by the disorder, and remedial steps are taken accordingly.

Frequently Asked questions about Sleep

Q: Why is sleep necessary?

A: Sleep is necessary because it ensures that the mind and body recharge, and also the immune system is strengthened. It also helps in building bone and muscle and performs tissue growth and regeneration functions. 

            A lack of sleep could lead to a lot of health-related issues. These include poor concentration, drowsiness, worsened physical performance, and even memory related issues.  If the lack of sleep persists over a long period, it could even lead to the individual getting mood changes and even hallucinations. Many sleep-experts states that the nervous system cells known as neurons are deprived of the energy content, or they could even get filled with cellular activity by-products. This happens because sleep in necessary for them to repair themselves and lack of sleep acts as a great impedance. Good quality sleep also ensures that proper connectivity is established within the entire nervous system.

Q: The new-borns require a lot of sleep as compared to adults! True or False?

A: True.

The newborns sleep anywhere from 10 to 18 hours in a single day. They have gaps between their sleep, and these could be as long as 1 to 3 hours. The gaps are spent by the newborns awake. It is also possible to witness the new-borns showing activity during sleep. They might suckle, smile, seem a bit restless, and even move their arms and legs.

Q: There are multiple stages of sleep. True or false?

A: True.

There are four stages of sleep. Some experts consider these to be five as the third stage is considered by them to be having two stages. The sleep cycle goes through the NREM stage first. It then enters the Rapid Eye Movement or REM stage.  The cycles repeat after the first REM sleep. Almost 50% of the sleep of a normal healthy individual is spent in stage 2 of the sleep while 20$ is spent in REM and the rest 30% in all the other stages of the sleep combined. The newborns spend close to 50% of their sleep time in the REM stage of sleep.

Q: Deprivation or lack of sleep could indicate several health issues. True or false?

A: True.

Deprivation of sleep or lack of sleep is indicative of several health-related issues.  The quality of sleep is also associated with health problems.  Problems related to mental illness, injury, poor quality of life, etc. would almost always have an impact on the nature of sleep or the quality of sleep. When an individualism deprived of his/her sleep, this could lead to many other subsequent issues like loss of work productivity, a rise in healthcare costs, etc. If the person is not able to sleep properly, health issues like obesity and depression are aggravated further.


1Sleep. (2019). Understanding Sleep Cycles: What Happens While You Sleep. Retrieved from

2Memar, P, F. F. (2018). A Novel, Multi-Class EEG-Based Sleep Stage Classification System. IEEE Trans Neural Syst Rehabil Eng., 84-95.

3NSF. (2019). Retrieved from National sleep foundation:

4Della Monica C, J. S. (2018). Rapid Eye Movement Sleep, Sleep Continuity, and Slow Wave Sleep as Predictors of Cognition, Mood, and Subjective Sleep Quality in Healthy Men and Women, Aged 20-84 Years. Front Psychiatry.

5Varga, B, G. A. (2018). Heart Rate and Heart Rate Variability during Sleep in Family Dogs. Animals (Basel).

8Mayo. (2019). Polysomnography (sleep study). Retrieved from Mayo Clinic:

6Patel, A. K., & Araujo, J. F. (2018). Physiology, Sleep Stages. NCBI Resources.

7Tuck. (2017). Sleep Spindles. Retrieved from Tuck:

9Labarca G, R. T. (2018). CPAP in patients with obstructive sleep apnea and type 2 diabetes mellitus: Systematic review and meta-analysis. Clin Respir J., 2361-2368.

10Lerche S, M. G.-S. (2018). Deterioration of executive dysfunction in elderly with REM sleep behavior disorder (RBD). Neurobiol. Aging, 242-246.

11Handley, S. (2017). Deformities of Nature: Sleepwalking and the Non-Conscious States of Mind in Late Eighteenth-Century Britain. J Hist Ideas, 401-425.

Scroll to Top