As most of us know, good sleep is essential for good health. Sleep is often explained as a state of reduced activity and reactivity of body and mind. The importance of sleep in regulating the basic functioning of human body and mind is beyond doubt.1
Sleep is generally explained by the behavior aspects of a person while asleep and the physiological changes that occur in the brain electrical rhythm in sleep. It is characterized by closed eyes, decreased response to external stimulation, species-specific posture, lack of mobility, increased reaction time and impairment cognitive function. The three major behavioral patterns in humans are wakefulness, non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). These three basic biological mechanisms have their independent impact on functions and controls of body.2
This article goes through the physiology of sleep, types, stages, importance of different stages of sleep, factors affecting the stages of sleep and adequate amount of sleep.
Types of sleep: There are two basic types of sleep, rapid eye movement (REM) and non rapid eye movement (NREM). Each one of these is characterized by specific brain waves and neuronal activity. During a typical night’s sleep, one cycles through all the stages of NREM and REM.
Phases/ stages of sleep:
While sleeping body cycles through 5 different stages of sleep. These stages are divided into wake, NREM and REM based on brain activity (electroencephalography), muscle tone (electromyography) and eye-movements (electro-oculography). In a healthy individual, during sleep body usually cycles through these stages on an average of 4-6 times, with each cycle having an average time of 90-110 minutes in each stage. NREM is further divided into three stages according to American Academy of Sleep Medicine (AASM) scoring manual as N1, N2, N3 (earlier N3 and N4) depending upon the physiology of brain and muscle activity.3 The first two cycles are dominated by slow wave sleep (SWS) and AASM stage N3, subsequent cycles contain less SWS and increased duration of REM sleep. Depending upon the stages of sleep, REM is characterized by more active and variable functions and less activity and stability during NREM sleep.2
Wake (Stage W): This is the first stage/ phase of sleep that is further characterized by open or closed eyes. During wakefulness or open eyes, brain activity shows alpha and beta waves, mostly beta. As the drowsiness increases and the eyes close, the brain activity is predominated by alpha wave pattern on EEG.
Non rapid eye movement sleep (NREM):
In a healthy adult, NREM sleep accounts for 75-80% of the sleep time. According to AASM on the basis of brain activity (EEG criteria), it is further divided into sub-stages N1,N2 and N3.3
N1: This is the lightest state of sleep, with body and brain activities to slow down with brief periods of movements. Stage 1 or N1 of NREM sleep occupies 3 to 8% of the sleep time and normally lasts just one to five minutes characterized by decrease in heart rate and brain activity. It is easy to wake up from the N1 stage of sleep but if left undisturbed one easily slides into deeper stages of sleep.4
N2: The next stage of NREM sleep is a period of light sleep before entering the deep sleep. Stage 2 or N2 of NREM sleep comprises 45- 55% of sleep time that lasts for about 25 minutes in the initial cycles and lengthens with each successive cycle. During this stage of sleep, the activity of brain slows down with occurrence of brain spikes up and down on EEG known as the sleep spindles. Sleep spindles are a hallmark of NREM sleep and their presence fulfils the criteria for onset of sleep. They manifest themselves as the transition of light sleep to deep sleep. Spindles occur during both light and deep slow waves of NREM sleep stage but not during the REM sleep. They are regularly generated about every 3 to 6 seconds. Sleep is characterized by inability to perceive or to react to external stimuli. Sleep spindles are partly responsible for this sensory shutdown, during a spindle formation transmission of sensory response from brain is suppressed and the response to external stimuli is minimized in sleep.5
N3 and N4 or N3: Stage 3 or N3 and N4 or N3 or SWS constitutes 15-20% of total sleep time. After about 30-60 minutes in N2 stage of NREM sleep, stage 3 or N3 begins. This is the final stage of NREM characterized by slow wave or delta sleep wave. During the N3 stage of NREM, heartbeat and breathing are at their slowest rate with no eye movements. The body is fully relaxed and tissue repair, growth and cell regeneration occurs with strengthening of immune system. It is difficult to arise from this stage of sleep. Most of the time spend during the deep sleep is in the first half of the night. During the early cycles of sleep, N3 stage accounts for 20-40 minutes of sleep, with the subsequent cycles these stages get shorter and more time is spent in REM sleep.4
REM (R): In healthy sleepers, this is the stage when the dreams happen. REM or stage R of sleep typically does not appear until approximately 90 minutes after the onset of sleep. It reoccurs after every 90 to 120 minutes with its distinct episodes but the duration increases as the night progresses. The first half of the night sleep contains less REM sleep than the second half.6 REM sleeps lasts approximately 10 minutes in the first cycle to usually lasting for roughly 60 minutes in the last cycle. During this stage, although the individual is behaviorally in sleep, but the brain and neuronal activity are high. This stage is characterized by the rapid eye movements, variable respiration, heart rate and presence of marked twitches in the limbs.7
The sleep cycle:
Sleep is not uniform throughout the night. In the course of the night, the total sleep consists of several rounds of alternating sleep cycles composed of individual stages of NREM and REM. On an average, a healthy sleeper on a typical night goes through four to six sleep cycles. In one night, all sleep cycles are not uniform, NREM and REM sleep continue to alternate throughout the night in a cyclic manner. In the first cycle of sleep, REM sleep is usually short lived i.e. 1 to 5 minutes. With the progress in night, in the second cycle of sleep decrease in time of stage 3 of NREM sleep occurs due the expanding stage 2 of NREM sleep and elongation of REM stage of sleep. REM sleep stage usually becomes longer with the progression of night. The average length of first NREM-REM sleep cycle is usually 70-100 minutes and the average length of second and later cycles is approximately 90 to 120 minutes. On an average, over the night the average period of NREM-REM cycle is approximated to be 90-110 minutes.8
The distribution of various stages of sleep varies throughout the night, in a young adult sleeper, SWS dominates the NREM portion of the cycle during the beginning of the night. Towards the second half of the night REM sleep episodes predominates. The preferential distribution of sleep towards the later portions of the night is thought to be associated with the circadian oscillation of the body’s regulatory mechanisms.8
Importance of sleep stages:
Sleep stages are important in terms of allowing body and brain to recuperate, refresh and develop. Failure to obtain enough NREM and REM sleep at night is associated with multiple health consequences. During normal sleep in adults, there is an orderly progression from wakefulness to onset of sleep to light or SWS sleep (NREM) to deep sleep (N3) then to REM sleep. Disturbances in different stage of sleep have profound adverse effects on the cognitive, physical, emotional and mental health of the individuals.2
Factors affecting the stages of sleep:
Age: This is the most strong and consistent factor that affect the pattern of sleep stages across the night in humans. The most marked changes related to age can be seen in newborns. In the first year of life, REM sleep constitutes the transition from wake to sleep. The pattern of cyclic NREM-REM sleep is present from birth but its duration is approximately 50 to 60 minutes. In young adults, SWS sleep is the predominant sleep, which decreases by nearly 40% in adults. With advancing age, a severe decline in SWS sleep has been recorded with well-maintained REM sleep patterns.8
Circadian rhythm: sleep stages are affected by circadian phase at which the sleep occurs. REM sleeps follows the circadian rhythm and peaks in the morning coinciding with the trough of the core body temperature rhythm. Thus, if the onset of sleep is delayed till the dawn, the reversal of normal sleep stage distribution is observed with sleep onset taking place with REM sleep disturbing the circadian rhythm of the body.8
Temperature: As discussed earlier REM sleep is sensitive to the core body temperature changes. Extremes of temperature in the sleeping environment are inclined towards causing disturbances in the sleep stages.8
Medication or sleep disorders: The distribution of sleep stages across the night are highly influenced by the use of medications and presence of sleep disorders.Use of anti anxiety medications tend to suppress SWS and certain serotonin uptake inhibitors affect the REM sleep. Similarly, presence of sleep disorders highly alter the NREM and REM cyclic patterns.8
Sleep apnea syndromes: These are the health issues related to difficulty in breathing while sleeping and are generally associated with the suppression of NREM sleep.It can increase daytime sleeping.8
Narcolepsy: narcolepsy is a nervous system disorder that causes excessive daytime drowsiness and sudden episodes of sleep. Narcolepsy is generally characterized by severe disturbances in the daily routines. Individuals suffering from narcolepsy usually present with disrupted circadian rhythms and disturbed sleep cycles.8
How much sleep is enough?
The need for sleep and the sleep patterns are regulated by the circadian rhythm, behavioral and environmental factors, which tend to vary over the course of human life. There is no fix number of hours that works for every individual for same age group. New borns initially sleep as much as 16 to 18 hours per day, which helps in boosting the growth and development especially the brain. School-age children and teenagers on average need about 9 to 10 hours of sleep per night. The need for sleep decreases with age, a healthy adult need an average sleep of 7 to 8 hours per night. With advancing age, nighttime sleep becomes lighter, shorter and disrupted by multiple factors.9
Some key points to generalize the functioning of sleep and stages of sleep:
- Sleep is formed by the cyclic pattern of NREM and REM stages
- Sleep is entered through NREM sleep stage
- On an average, NREM and REM sleep alternate with a period of approximately 90 minutes
- SWS or N3 sleep predominates in the first third of the night associated with initiation of sleep and wake time.
- The last quarter of the night is predominated by the REM sleep and is associated with circadian rhythm and thermoregulation
- Wakefulness in sleep accounts for less than 5% of the total sleep
- Stage 1 or N1 usually constitutes 2% to 5% of the total sleep at night.
- Stage 2 or N2 usually constitutes 45% to 55% of the total sleep at night
- Stage 3 or N3 or SWS usually constitutes 15% to 20% of the total sleep at night
- Therefore, NREM sleep constitutes 75% to 80% of the total sleep
- Stage R or REM sleep constitutes 20% to 25% of the total sleep occurring in four to six discrete episodes.
FAQs on sleep and stages of sleep:
Q1. Why do we sleep?
A1. Sleep is an important activity to ensure proper functioning of the various systems of the human body. A good quality adequate sleep helps in maintaining good mental and general health. Insufficient sleep can lead to multiple health issues such as drowsiness, poor concentration, memory function and impaired physical performance.
Q2. How much sleep is enough?
A2. There is no generalized answer for this question. The quantity of sleep varies with age and health. On an average a healthy adult should sleep for at sleast 7-8 hours at night.
Q3. How many stages are there in one sleep cycle?
A3. Every sleep cycle is formed of 5 stages of sleep varying in length and benefits. Stage W is the wake stage of the sleep, this is the awake time in the bed before falling asleep. Stage 1 and 2 are known as the stages of light sleep. During this stage, with the initiation of sleep cycle, muscles start to relax, breathing and heart rate slows down, body temperature drops and transition to next stage i.e stage 3 and 4 or N3 happens. Then the last stage of sleep cycle occurs i.e. stage R or REM stage of sleep.
Q4. How much time is spent in different stages of sleep?
A4. Although the time spent in different stages of sleep varies with the progression of night. On an average, the time spent in stage W or awake stage is 2-5%, followed by 45-55% in the light sleep or stage N1 and N2, then 15-25% in deep sleep or N3 and N4 and 20-25% in REM sleep.
Q5. Which is the deepest and most beneficial stage of sleep?
A5. Stage N3 & N4 or stage N3 is the deepest stage in the sleep. In this stage, blood flow to muscles increases, with further drop in blood pressure, heart and respiratory rate, there is increase in the production of repair and growth hormones. It is the most rejuvenating and restorative stage of sleep, facilitating muscle growth, repair of tissues and removal of waste from brain and body.
Q6. What is the duration of normal sleep cycle?
A6. During an ideal night, a healthy individual goes through several 90-minute alternating cycles that sample each stage of sleep. Every alternating cycle plays an important role in maintaining mental and physical well-being. The amount of each stage of sleep varies significantly in each cycle with the progression of night.
Q7. How many cycles of alternating sleep stages are there in one night?
A7. In the course of the night, the total sleep consists of several rounds of alternating sleep cycles composed of individual stages of NREM and REM. On an average, a healthy sleeper on a typical night goes through four to six sleep cycles.
Q8. Which stage of sleep is associated with learning and memory functioning?
A8. Learning and memory are mostly described by three functions, introduction of new information, stabilization of memory and ability to recall the information. Several studies have documented the role of REM stage of sleep in learning and consolidation of memory.
Q9. Why do we wake up at 3 am?
A9. People generally wake up around 3am in the night because around this hour of night shift from deep sleep to lighter sleep takes place. For instance, if you go to bed around 11 at night, by 3am you are mostly out of deep sleep and shifting to stages of lighter sleep i.e. REM in which brain is more active and it is more likely to be awaken.
Q10. How important is a good night’s sleep?
A10. Sleep regardless of age, gender or race is extremely important for health and general well-being. It is a basic and fundamental requirement of a human body to restore its functioning. During the sleep, tissues grow and repair itself and strengthen the immune system. The brain also rejuvenate and repair to maintain performance functioning. Last but not the least sleep regulates the release of various hormones in body and maintains the proper functioning of metabolic, growth, cardiac, nervous system of the body.
Sleep is a period of reduced activity or alertness regulated by the body’s internal clock, helping the physical and mental state of an individual to recover and refresh. The amplitude of changes seen in brain and neuronal activity during sleep exceeds those, which occur during most waking periods. Sleep is a phasic biological mechanism, mandatory to maintain the well-functioning of the body. To conclude, each night our body goes through alternating cyclic pattern of five stages of sleep. During these stages our breathing, heart rate, muscles and brain waves are well regulated and affected differently. Getting enough sleep at night is very important for the basic functions like digestion, growth, learning and memory consolidation. Hence, it is important to maintain good sleep habits in order to attain a quality sleep and general health well-being.
- Buysse DJ. Sleep health: can we define it? Does it matter?. Sleep. 2014;37(1):9-17. doi:10.5665/sleep.3298
- Chokroverty, S. (2009). An Overview of Normal Sleep. Sleep Disorders Medicine. 2009; 5–21.doi:10.1016/b978-0-7506-7584-0.00002-1
- Moser D, Anderer P, Gruber G, Parapatics S, Loretz E, Boeck M, Kloesch G, Heller E, Schmidt A, Danker-Hopfe H, Saletu B, Zeitlhofer J, Dorffner G. Sleep classification according to AASM and Rechtschaffen & Kales: effects on sleep scoring parameters. 2009;32(2):139-49. doi: 10.1093/sleep/32.2.139.
- Patel AK, Reddy V, Araujo JF. Physiology, Sleep Stages. [Updated 2020 Apr 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls https://www.ncbi.nlm.nih.gov/books/NBK526132/
- Schönauer M, Pöhlchen D. Sleep spindles. Curr Biol. 2018;28(19):R1129-R1130. doi: 10.1016/j.cub.2018.07.035.
- Keenan S & Hirshkowitz M. (2011). Monitoring and staging human sleep. In Kryger MH, Roth T &Dement WD (Eds.), Principles and practice of sleep medicine, 5th edition, (pp 1602-1609). St. Louis: Elsevier Saunders.
- Siegel JM. Clues to the functions of mammalian sleep. Nature 2005;437(7063):1264-71. doi: 10.1038/nature04285.
- Carskadon MA & Dement WC. (2011). Monitoring and staging human sleep. In Kryger MH, Roth T &Dement WD (Eds.), Principles and practice of sleep medicine, 5th edition, (pp 16-26). St. Louis: Elsevier Saunders.http://apsychoserver.psych.arizona.edu/JJBAReprints/PSYC501A/Readings/Carskadon%20Dement%202011.pdf
- Hirshkowitz M, Whiton K, Albert SM, Alessi C, Bruni O, DonCarlos L, Hazen N, Herman J, Adams Hillard PJ, Katz ES, Kheirandish-Gozal L, Neubauer DN, O’Donnell AE, Ohayon M, Peever J, Rawding R, Sachdeva RC, Setters B, Vitiello MV, Ware JC. National Sleep Foundation’s updated sleep duration recommendations: final report. Sleep Health. 2015;1(4):233-243. doi: 10.1016/j.sleh.2015.10.004.