Physiology of Sleep

Sleep is a reversible phase of diminished responsiveness, motor activity, and metabolism. This process is a complex and dynamic phenomenon, occurring in 4–5 cycles a night, and generally divided into non–rapid eye movement (NREM) sleep and REM sleep stages. NREM sleep, consisting of 3 stages, represents the period of transition from wakefulness, somnolence, and then deep sleep. REM sleep is characterized by rapid irregular eye movement, but with voluntary muscle atonia. The sleep stages and associated disorders are studied through the use of polysomnography, a multimodal test evaluating the brain waves, along with ocular and motor activities. Sleep disorders can be primary (which do not stem from any health problem) or can be attributable to underlying conditions or medications.

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Sleep is a reversible phase of diminished responsiveness, motor activity, and metabolism. The purpose of sleep is poorly understood; however, there are multiple theories:

  • Survival theory: 
    • Humans are diurnal by nature (creatures of the day). 
    • Awake during the day because there is better vision and fewer predators
  • Restorative theory: 
    • Sleep allows the body to heal and repair tissue, including brain reorganization, consolidation, and storage of memories. 
    • Sleep also functions as stress relief. 
  • Growth theory
    • During sleep, the pituitary gland releases growth hormones.


  • Characteristics of sleep in healthy persons include:
    • Closed eyes
    • Reduced brain responsiveness to stimuli
    • Differences in heart rate, BP, and respiration depending on the stage of sleep
  • Circadian rhythm: physiologic changes (including physical, mental, and behavioral) that follow a 24-hour cycle, primarily responding to light and dark
  • Suprachiasmatic nucleus (SCN, the body’s master clock):
    • Found in the hypothalamus
    • Responsible for controlling circadian rhythms and the sleep cycle
    • Regulated by the environment, receiving input from the eyes (light)
    • Decrease in light → stimulation of SCN → release of norepinephrine → stimulation of pineal gland → release of melatonin → sleep 
    • The influence of circadian rhythm is also seen in hormones (with predominant nocturnal secretion) such as:
      •  Prolactin
      • Testosterone
      • Growth hormone

Stages of Sleep

  • Typically 4–5 cycles in an 8-hour period (1 sleep cycle: 90–120 minutes)
  • Divided into non–rapid eye movement (NREM) and REM sleep stages
  • The amount of sleep as well as duration of each stage of sleep change as humans age.

Non–rapid eye movement (NREM) sleep

  • Further divided into:
    • Stage N1 (5%–10% of total sleep cycle)
    • Stage N2 (45%–55% of total sleep cycle)
    • Stage N3 (10%–20% of total sleep cycle)
  • Physiologic changes:
    • ↑ Parasympathetic tone
    • Low BP, slow heart rate, ↓ respiratory rate
    • Less vivid dreams
    • Muscle tone similar to wakefulness

REM sleep

  • Stage R sleep
  • Usually begins 90 minutes after onset of sleep and averages 4–5 episodes/night
  • Duration increases throughout the night, with increased percentage during the latter half of the night.
  • Constitutes 20%–25% of the total sleep duration
  • Believed to have a role in memory consolidation
  • Physiologic changes:
    • Surges of sympathetic and parasympathetic activity
    • Rapid irregular eye movement occurs because of activity of paramedian pontine reticular formation (PPRF), which is the conjugate gaze center.
    • Transient increase in BP and respiration
    • Variability in heart rate 
    • Penile/clitoral erection (due to increased blood flow)
    • Vivid dreams
    • Absent muscle tone
Stages of healthy sleep

Stages of healthy sleep:
Sleep is divided into non–rapid eye movement (NREM; blue bars) and REM (orange bars) stages in each sleep cycle. Around 4–5 sleep cycles occur throughout the night, with each cycle averaging 90–120 minutes.

Image by Lecturio.

Study of Sleep


  • Also known as a sleep study, a multimodal test evaluating processes in sleep:
    • EEG: measures brain electrical activity
    • Electro-oculography (EOG): measures eye movements
    • Electromyography (EMG): measures muscle movements 
  • Important parameters:
    • Sleep latency: period of time to transition from “lights out,” or initiation of sleep, to onset of sleep 
    • REM latency: period of time from initiation of sleep to reaching 1st REM stage
    • Total sleep time: amount of sleep time during the recording time
    • Sleep efficiency: percentage of total sleep time over total time in bed 
    • Wake after sleep onset: period of wakefulness after onset of sleep
    • Wake after sleep offset: period of wakefulness after sleep offset (i.e., long wakeful time after an atypical early morning awakening may be a sign of depression)
    • Apnea index: measures apneic episodes ≥ 10 seconds per 1 hour of sleep
    • Nocturnal myoclonus index: measures number of periodic leg movements per hour

Awake and sleep stages

Generally, an adult is awake for ⅔ of the day. The body relaxes before going through different stages of sleep.

Table: Stages of sleep
StageDescriptionEEG wave
Awake Eyes open Person is alert and active. Beta waves
Eyes closed
  • Decrease in level of alertness
  • Relaxation
Alpha waves
NREM sleep Stage N1
  • Transition from wakefulness
  • Somnolent/drowsy
  • Light sleep
Theta waves
Stage N2
  • Less responsive, deeper sleep
  • Heart rate and temperature decrease
  • Longest stage by total duration
  • Theta waves
  • Sleep spindles (bursts of monomorphic 12- to 14-Hz waves)
  • K complexes (negative sharp waves, followed by positive component)
Stage N3
(deep sleep, or slow-wave sleep)
  • Restful sleep, deepest NREM stage
  • More difficult to arouse
  • Body/tissue growth and repair occurs.
  • Decreases with age
Delta waves
REM sleep
(stage R)
REM sleep
(dream state)
  • Mixed, low-voltage pattern
  • Sawtooth waves
  • REM
  • Irregular, sharply peaked eye movements
  • Atonia of voluntary muscles (except eye and diaphragm)
  • Inactivity due to inhibited alpha motor neurons
Beta waves
(similar wave to awake state, but with atonia)


The following mnemonic summarizes the sleep waves in the different stages of sleep:
“At night, BATS Drink Blood”

  • B: beta (awake, eyes open)
  • A: alpha (awake, eyes closed)
  • T: theta (stages N1 and N2)
  • S: sleep spindles (stage N2)
  • D: delta (stage N3)
  • B: beta (REM)
Sleep stages and EEG waves

Sleep stages and EEG waves:
Stage N1 shows theta waves.
Stage N2 also has theta frequency but has distinct features: sleep spindles and K complexes.
Stage N3 (deep sleep) is characterized by delta waves.
During REM sleep, sawtooth waves (beta waves similar to wakeful state) are seen, with electromyogram exhibiting atonia.

Image by Lecturio.

Sleep and Associated Disorders

Sleep-associated disorders

NREM sleep:

  • Related disorders:
    • Sleepwalking
    • Night terrors
    • Confusional arousals
    • Sleep-related eating disorder (sleep eating)
  • Bedwetting (enuresis) 

REM sleep:

  • Related disorders:
    • Narcolepsy
    • Nightmares
    • REM sleep behavior disorder
  • Bedwetting (enuresis)

Conditions associated with sleep problems

  • Depression:
    • Increased REM sleep
    • Decreased REM sleep latency
    • Decreased N3
    • Repeated nighttime awakenings
    • Early morning awakening (terminal insomnia)
  • Pulmonary disease and obstructive sleep apnea (OSA):
    • Reduced N3 in OSA
    • REM (with atonia) affects those dependent on intercostal and accessory respiratory muscles for ventilation → hypoventilation and desaturation

Substances and associated sleep changes

Table: Substances and associated sleep changes
Selective serotonin reuptake inhibitors
  • Increased N1
  • Reduced total sleep
  • Increased N2
  • Reduced REM sleep
  • Increased N3
  • Reduced REM sleep
  • Reduced N3
  • Reduced REM sleep
Monoamine oxidase inhibitors
  • Reduced N3
  • Reduced REM sleep
Tricyclic antidepressants
  • Reduced N3
  • Reduced REM sleep
  • Reduced N3
  • Reduced REM sleep

Clinical Relevance

  • Circadian rhythm sleep–wake disorder: recurrent patterns of sleep disruption due to abnormalities of the circadian system or when the sleep–wake cycle is misaligned with the environment. Subtypes include delayed sleep-phase disorder, advanced sleep-phase disorder, shift-work disorder, and jet-lag disorder.
  • Parasomnias: pattern of sleep disorder marked by unusual behavior, movements, or experiences that occur during sleep or sleep–wake transitions. Symptoms may include abnormal movements, emotions, dreams, and autonomic activity.
  • Restless legs syndrome (Willis-Ekbom disease): sleep-related movement disorder that is marked by an overwhelming urge to move the legs accompanied by an unpleasant sensation that is relieved with movement. Symptoms occur during the evening and are associated with sleep disturbance. 
  • Narcolepsy: sleep disorder characterized by excessive daytime sleepiness and falling asleep at inappropriate times. Narcolepsy is associated with hypnagogic hallucinations (vivid tactile/auditory or visual hallucinations), cataplexy (emotionally triggered loss of muscle tone, during laughing, for example), and sleep paralysis (inability to move after awakening).
  • Insomnia: disorder of sleep difficulty (problems with initiation and/or maintenance of sleep) marked by symptoms that interfere with duration and/or quality of sleep despite adequate opportunity for sleep.
  • Obstructive sleep apnea: episodic apnea, or cessation of breathing during sleep, in which the period of apnea lasts for ≥ 10 seconds. Obstructive sleep apnea is usually due to a partial or complete collapse of the upper airway and is associated with snoring, restlessness, daytime headache, and somnolence.


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  2. Kirsch, D. (2020). Stages and architecture of normal sleep. UpToDate. Retrieved March 27, 2021, from
  3. Kotagal, S. (2021). Parasomnias of children, including sleepwalking. UpToDate. Retrieved May 16, 2021, from
  4. Patel A.K., Reddy, V., Araujo, J.F. (2020). Physiology, Sleep Stages.
  5. Roehrs, T., Roth, T. (2021). The effects of medications on sleep quality and sleep architecture. UpToDate. Retrieved May 16, 2021, from
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  7. Shrivastava, D., Jung, S., Saadat, M., Sirohi, R., Crewson, K. (2014). How to interpret the results of a sleep study. Journal of Community Hospital Internal Medicine Perspectives 4:24983.

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