Humankind, like all living organisms, is an adaptive species that has evolved both voluntary and involuntary processes concurrent with the surrounding physical environment. We take our cues from the climate, lunar and solar cycles to create an internal clock or circadian rhythm .

Sleep and wakefulness are set in a circular pattern over a 24-hour period in conjunction with brightness intervals during the day and darkness at night. We are a diurnal species; that is, we are active during the day, and we sleep at night.

Our brains, the control centers of our physical, instinctual, and emotional bodies, sense outside light or darkness and secrete hormones that either put us to sleep or keep us awake accordingly. For example, when daylight fades to night, the pineal gland secretes the hormone melatonin, making you sleepy.

Sleep may appear as an outwardly calm and simplistic state of deactivation; however, a growing number of studies reveal how complex and vital sleep really is. Here we will describe the five stages of the sleep cycle, explaining each stage’s internal processes, function, and importance. We will also give a brief overview of each sleep cycle’s duration and how sleep cycles change with age.

Importance of Sleep

Sleep is a restorative process that we need for development, information processing, and overall functioning in our waking lives. Sleep is paramount to longevity, immune health, physical growth and development, emotional health, and mental acuity. Sleep basically gives us a chance to process and consolidate information we gather while awake and restores our physical, mental, and emotional condition.

Studies have shown that people who get adequate sleep live longer and get sick less than people who do not get enough sleep. Sleep is also a crucial part of brain and musculoskeletal development in infancy and childhood. Sleep also helps regulate hunger, mood, and physical vivacity.

Lack of sleep causes us to be hungry in order to stay awake, so we overeat, which leads to deadly conditions like heart disease, diabetes, and strokes. Lack of sleep is also linked to irritability, confusion, and anxiety, while too much sleep correlates with depression.

Stages of Sleep

There are five stages of sleep in each sleep cycle. One sleep cycle is completed every 70-120 min in healthy adults and consists of 5 stages. An individual thus completes 2-4 sleep cycles per night, although three cycles are ideal.

Furthermore, these five stages fit into one of two categories: NREM or REM. REM stands for Rapid Eye Movement sleep, while NREM is Non-Rapid Eye Movement. The first four stages of the sleep cycle are NREM, with the final stage being REM.

Before describing each stage of sleep in the sleep cycle, it is essential to understand brain waves, as they are a form of measurement and a key indicator of sleep stages and wakefulness. Beta and Gamma waves are the daytime brainwaves with high (fast) frequencies involved in cognitive efforts and conscious perception.

Generally, faster brain waves occur when we are awake, and slower brain waves occur during sleep. Alpha, Theta, and Delta waves occur during the sleep cycle, depending on which stage you are in:

  • Alpha Waves: These are the fastest waves we experience while we sleep, but we also experience them while we are awake. They are related to thinking, feeling, communication, sleep, and general body function.
  • Theta Waves: These are the slower brain waves associated with the first two stages of NREM sleep.
  • Delta Waves: There are the slowest brain waves that occur in the most profound sleep state.
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Sleep cycles must complete all five stages to fully rest and replenish all vital physical and mental functioning. Every stage has a particular form and function that both perpetuate the sleep cycle and collectively process, integrate, and restore brain power and physical energy and functionality.

Stage 1: NREM

Stage 1 is the beginning stage of sleep and lasts 1-7 minutes. During this stage, faster alpha waves dissipate as slower theta waves appear. This first stage is known as the “nodding off” period of limbo between awake and asleep.

During this stage, breathing and heart rate decelerate, and blood pressure and brain temperature decrease in preparation for a deeper sleep. Muscles still move, so many in this state may bob their heads, roll their eyes, or sleep with the eyes partially open.

Stage 1 is considered light sleep, and if woken from this state, you will not feel rested at all. Muscle twitching and hypnic jerks are also characteristic of stage 1. Hypnic jerks refer to abruptly jolting awake, which certain scientists theorize to be evolutionary remnants of protective measures that saved early homo sapiens from fallout out of trees.

Stage 2: NREM

Stage 2 of the sleep cycle lasts 10-25 minutes and can be described as the processing and integrating stage of sleep. During this stage, the brain processes and integrates memories and emotions. Two phenomena unique to this stage are sleep spindles and K-complexes.

Sleep spindles are quick surges of high-frequency brain activity, while K-complexes are a type of brain wave that suppresses response reactions to outside stimuli. Together, K-complexes and sleep spindles function to consolidate and process the day’s sensory and emotional memories.

Maintenance of digestive functioning and metabolism also occurs during this phase. While heart rate, body temperature, blood pressure, and cell activity continue to decrease during stage two, brain waves still oscillate at a rate fast enough to be considered light sleep. However, it is harder to wake up from stage two sleep, and brain waves begin to slow down significantly in preparation for stages 3 and 4.

Stage 3 and 4: Deep sleep, NREM

Stage 3 and 4 are known as deep sleep or delta sleep as they are characterized by the presence of delta brainwaves, slow rhythmic breathing, a slow heart rate, and no muscle movement. This stage lasts between 20-40 minutes and is the most restorative stage for your physical body.

The difference between stage 3 and stage 4 of deep sleep is the proportion of delta waves present. During stage 3, brainwave activity contains between 20 and 50 percent, while stage 4 shows delta waves accounting for over 50 percent of brain activity. It is worth noting that many sleep experts now combine the two stages into one stage, or stage 3. 

Dreaming can occur during deep sleep, but dreams are less common, less memorable, and less intense than in REM sleep. Dreams may be recapitulations of mundane events that we easily forget upon waking. Brain activity is minimal; however, there is the potential for dissociated sleep states, or parasomnias,  like sleepwalking and night terrors. 

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The primary function of deep sleep is to restore, rebuild, and repair cellular, physiological, and immune function. Studies show that the brain releases human growth hormone (HGH) during deep sleep, which aids in musculoskeletal development in growing children and regulates metabolism, muscle and bone growth, and body composition in adults.

Stage 5: Rapid Eye Movement (REM)

REM sleep accounts for 20-25 percent of the sleep cycle but varies in duration with each cycle, increasing in duration with each subsequent cycle. During REM sleep, brain waves take on a fast alpha rhythm, heart rate accelerates drastically, and breathing is quick and shallow. True to its name, our eyes move rapidly under closed lids during REM, and we often have vivid and strange dreams.  

Many regions of the brain activate during REM to achieve our dreams. The thalamus starts to deliver sensory stimuli to the frontal cortex, which gives imagery, sound, and sensation to our dreams. The brainstem sends signals that paralyze our arms and legs to prevent us from physically manifesting or “acting out” our dreams.

This paralysis can also play out in your dreams; perhaps you’ve experienced the frustration of spaghetti legs while trying to escape a chase or throwing a punch that feels like it turns to mush on impact.

Lastly, a structure in the brain called the amygdala becomes highly active during REM. The amygdala serves to process emotions. Thus, REM is incredibly crucial for brain function, memory, and emotional health, but also maintains body function with cell protein production.

An excess of REM sleep correlates with depression to such an extent that a critical mechanism in anti-depressant medication is REM blockage. Too little REM sleep is associated with memory loss, disorientation, grogginess, and slowed perception. Procrastinators should beware of pulling an all-nighter because the age-old adage of getting a good night’s sleep before a test holds true.

How Many Hours of Sleep Do You Need?

Medical experts recommend 7-9 hours of sleep during a 24-hour period for adults. Infants, children, and adolescents require more sleep to aid brain and skeletal development; infants require 12-16 hours of sleep, children require 10-12 hours of sleep, and adolescents need 9-11 hours of sleep. Both too little and too much sleep have harmful side effects.

Sleep deprivation symptoms include tiredness, anger, confusion, lack of concentration, and in severe cases, hallucinations, psychotic episodes, and death. Sleeping too much could result from underlying medical conditions such as hypothyroidism or iron and B complex deficiencies, and have been linked to obesity, heart disease, depression, and increased mortality rates.

While napping may not be customary in the US, they are beneficial for our bodies and minds. According to studies, power naps that last from 5 to 20 minutes are physically restorative. Naps 90 minutes in duration allow for a full sleep cycle, including REM, which helps restore memory and brain function. In many cultures, people sleep 5-6 hours at night with an hour-long nap after lunch to compensate.

Sleep Cycles

As stated, we typically pass through three 90-120-minute sleep cycles, but each cycle is distinct as there are different proportions of each stage of sleep in every cycle. The amount of time spent in REM sleep in our first cycle lasts 10 minutes; however, the REM stage increases with each subsequent cycle.

Consequently, the duration of deep sleep decreases with each cycle. Furthermore, the duration and proportion of stages in cycles differ depending on age. (Harvard med school) For example, babies’ cycles last 50-60 minutes, the majority of which consist of active (REM) sleep.

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Children and adults spend 20-25% of total sleep in the REM stage but differ on the amount of time spent in deep sleep, with children spending a much higher percentage of time in deep sleep than adults. Children and adolescents need more deep sleep to facilitate physical and mental growth.

At puberty, our circadian rhythms shift, causing us to stay up and sleep in later. Mid to late adulthood sees a shift back to an earlier internal clock, less time spent in deep sleep, and more time spent in light sleep.


  1. How many stages are there in a sleep cycle?
    • There are five stages. Stages 1-4 are Non-Rapid Eye movement sleep (NREM), and stage 5 is Rapid Eye Movement sleep (REM).
  1. What are the characteristics of each stage?
    • Stage 1- Light sleep, theta waves, hypnic jerks, slowing heart rate, breathing, and muscle movements.
    • Stage 2- Light sleep, sleep spindles, k-complexes, processing and integrating, memory consolidation, continued slowing of vital functions, continued deceleration of brainwaves.
    • Stage 3 and 4- Deep sleep, presence of Delta waves, mundane dreams, lack of brain activity, the release of HGH, body, immune system restoration, metabolic regulation, parasomnias
    • Stage 5- REM sleep, alpha waves, vivid and intense dreams, rapid eye movement, fast heartbeat, rapid and shallow breathing, a drastic increase in brain activity over various regions, regulates emotions, restores memory.
  1. How long do sleep cycles last?
    • Sleep cycles last 90-120 minutes for children and adults
    • Sleep cycles last 50-60 minutes for infants
  1. How many hours of sleep should you get per 24-hour period?
  • Adults- 7-9 hours
  • Adolescents 9-11 hours
  • Children 10-12 hours
  • Infants 12-16 hours
  1. What are the benefits of naps?
    • Naps help compensate for lack of sufficient sleep cycles during the night. Short, power naps help restore body function, while 1-1.5-hour naps supply a full REM cycle for brain and memory function restoration.
    • Many cultures shut down business for two hours after lunch for daily napping periods. In Spain, these periods are called “siestas.”


Sleep is a multi-faceted and adaptive process described by cyclical stages that serve a distinct purpose in developing and restoring our physical, emotional, and mental bodies. Far from inert and inactive, our brains come alive during the sleep cycle, orchestrating deliberately timed messages that signal the end of one stage and the beginning of another.

During the beginning stages of sleep, we begin to slow our breathing, heart rate, and muscle movements while decreasing heart rate, blood pressure, and brain wave frequency in preparation for deep sleep. We also begin to consolidate, process and integrate sensory and cognitive information received over the previous day.

During deep sleep, vital functions and brainwave frequencies reach an all-time low, and the brain secretes growth hormones to restore and regulate our physical bodies. The last stage of sleep is the REM cycle, which increases our vital functions, causing a mysterious and emotional dream state that replenishes our memory and regulates our emotions.

The sleep cycle repeats itself fluidly, as REM sleep subsides with the gradual deceleration of light sleep. We see cyclic rhythms in every living organism. Like all cycles inherent in nature, the sleep cycle represents the power of adaptation and the interconnectedness of our internal and external worlds.

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