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Sleep and sleep disorders

Last updated: September 21, 2021

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Sleep is a physiologically recurring state of rest characterized by relative suspension of consciousness and inaction of voluntary muscles. It is regulated by the circadian rhythm and usually consists of 4–5 sleep cycles that include three stages of non-rapid eye movement sleep (NREM sleep) and one stage of rapid eye movement sleep (REM sleep). Sleep disorders can be grouped into primary (i.e., due to an intrinsic disorder of the sleep-wake cycle) and secondary (i.e., due to an underlying medical condition). Primary sleep disorders are further divided into dyssomnias and parasomnias. Symptoms include difficulty falling asleep, difficulty remaining asleep, or abnormal behavior during sleep. Treatment may include sleep hygiene practice, phototherapy, and sedative pharmacotherapy.

Narcolepsy, restless legs syndrome, and obstructive sleep apnea are discussed in separate articles.

Normal sleep cycle [1]

Normal sleep phases [1][3]

Sleep phases characteristics
Phase Description EEG findings Sleep-phase-specific disorders
  • Beta waves (> 13 Hz) with the lowest amplitude
  • Eyes closed
  • Alpha waves (8–13 Hz)

NREM sleep

Stage N1
  • Lightest stage of sleep
  • 5–10% of total sleep time
  • Characterized by hypnic jerks: spontaneous myoclonic contractions associated with a sensation of twitching or falling
  • Theta waves (4–7 Hz)
Stage N2
  • Deeper sleep compared to N1 sleep
  • 45–55% of total sleep time
  • Theta waves (4–7 Hz)
  • Sleep spindles: short bursts of EEG waves with a frequency of 12–14 Hz [4]
  • K-complexes: high amplitude, diphasic, frontocentral slow waves
  • Teeth grinding (bruxism) most commonly occurs during N2 sleep.
Stage N3
  • Slow-wave sleep (deepest sleep)
  • 10–25% of total sleep time
  • The percentage of N3 stage decreases with age.
  • The length of N3 stages decreases over the course of the night.
  • High waking threshold
  • Delta waves (EEG) (0–4 Hz) with the highest amplitude

REM sleep

  • General characteristics
    • 18–25% of total sleep time
    • High dream activity
    • The percentage of REM sleep decreases with age.
    • The duration and the proportion of REM sleep in relation to NREM sleep increases over the course of the night.
    • High waking threshold
  • Electromyographic activity
  • Autonomic activity
  • REM sleep latency
    • Period of time between the onset of sleep and the first REM episode
    • Typically lasts 90–120 minutes in a healthy person

To remember the sequence of brain wave changes during sleep, think: “For BETTER (read “beta”) WAVES, Ask The Silent Surfer Dozing at the Beach” (Beta waves while awake; Alpha waves with eye closure; Theta waves during N1; Sleep spindles during N2; Delta waves during N3; Beta waves during REM sleep).

To remember when teeth grinding (N2 stage) and bedwetting (N3 stage) occur during NREM sleep, think: “I saw the t(w)ooth fairy fleeing and the three little pigs peeing”.

Primary sleep disorders


Parasomnias [6]

Secondary sleep disorders

  • See “Sleep-phase-specific disorders” in “Sleep physiology” section above.

Common features of circadian rhythm sleep-wake disorders

Delayed sleep phase disorder

Advanced sleep phase disorder

Jet lag disorder

Shift-work disorder

Non-24 hour sleep-wake disorder [9]


  • Prevalence: ∼ 20% of the population worldwide (most common sleep-wake disorder) [10]
  • Etiology
  • Classification
    • Acute or transient (< 3 months): associated with stress or a disrupted sleep schedule
    • Chronic (≥ 3 months): associated with an increased risk of psychiatric illness and functional impairment

Clinical features

  • Initial or sleep-onset insomnia: difficulty initiating sleep
  • Middle or sleep-maintenance insomnia: frequently waking from sleep
  • Late or sleep-offset insomnia: awakening early in the morning
  • Nonrestorative sleep: feeling fatigued after waking

DSM-5 diagnostic criteria [11]

  • Problems initiating or maintaining sleep, or awakening early in the morning and being unable to return to sleep
  • Symptoms occur ≥ 3 days/week for ≥ 3 months
  • Symptoms cause functional impairment or distress
  • Symptoms are not caused by an underlying substance or medication use
  • Symptoms occur despite having enough time to sleep
  • No underlying or coexisting psychiatric or medical disorder that explains symptoms


Behavioral therapy

  • Sleep hygiene
    • No alcohol 4–6 hours preceding sleep
    • No stimulants: Caffeinated drinks and nicotine should be avoided 3–4 hours before bedtime.
    • Regular exercise is beneficial but should be avoided 6 hours before bedtime.
    • Quiet, dark, pleasantly cool bedroom and a comfortable bed
    • No large meals before bedtime
  • Stimulus control therapy: Insomnia disorder may cause the bed and bedroom to become cues for arousal rather than sleep. Stimulus control instructions aim to correct this by re-establishing the association of the bed and bedroom with sleep.
    • Advise waking up at regular times (also during the weekend and holidays).
    • Discourage engaging in other activities in bed such as working or reading.
    • Leave the bedroom when unable to fall asleep within 20 minutes (e.g., to read or listen to music) and return only when sleepy.
    • Advise against afternoon naps; if taken, this should not take place after 3 p.m. and naps should be no longer than 1 hour.
  • Sleep restriction therapy
    • A cognitive behavior therapy for patients with chronic insomnia, where the amount of time spent in bed is restricted to their average estimated sleep time.
    • When sleep efficacy (total sleep divided by time spent in bed) is greater than 90%, the amount of time spent in bed is increased.
    • This has been shown to reduce sleep latency.
  • Cognitive-behavioral therapy (CBT): preferred treatment for chronic insomnia



Clinical features

  • Excessive sleep (with decreased sleep quality)
  • Difficulty awakening from sleep
  • Sleep inertia (impaired alertness or excessive fatigue after waking)
  • Automatic behaviors (with no memory of the episode after waking)

DSM-5 diagnostic criteria [13]

  • Excessive sleepiness despite ≥ 7 hours of sleep with at least one of the following:
    • Recurrent periods of sleep on the same day
    • > 9 hours of sleep that is nonrestorative
    • Impaired alertness after awakening
  • Symptoms occur ≥ 3 days/week for ≥ 3 months
  • Symptoms cause functional impairment or distress
  • Symptoms not caused by an underlying substance or medication use
  • Symptoms occur despite having enough time to sleep
  • No underlying or coexisting psychiatric or medical disorder that explains symptoms


Sleepwalking disorder

Sleep terror disorder

Nightmare disorder

REM sleep behavior disorder

Other parasomnias

To remember that nightmare disorder occurs during REM sleep and the experience is remembered, while sleep terror disorder occurs during non-REM sleep and is not remembered, think: I REMember my NIGHTMARE, and there were NO memorable TERRORists.

Normal changes in sleep architecture occur with aging, and include:

  • Decreased total sleep time [21]
    • Decreased time spent in deep sleep and REM sleep
    • Increased sleep latency
    • More frequent nighttime awakenings that are likely multifactorial (e.g., due to nocturia, pain, and/or less time spent in deeper stages of sleep)
  • Advanced circadian rhythms resulting in earlier bedtimes and thus morning awakenings [21]
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