Sleep physiology

Non-REM (NREM) Sleep

• N1 (Light Sleep)

  • EEG: Theta waves.
  • Description: Transition from wakefulness to sleep (5% of total sleep). Very light sleep from which one is easily awakened.
  • Physiology: Slow, rolling eye movements.

• N2 (Intermediate Sleep)

  • EEG: Sleep spindles and K-complexes on a background of theta waves.
  • Description: Largest percentage of total sleep time (~45-55%). Deeper sleep than N1.
  • Key Associations: Bruxism (teeth grinding) often occurs during this stage.

• N3 (Deep Sleep / Slow-Wave Sleep)

  • EEG: Delta waves (low frequency, high amplitude).
  • Description: Deepest stage of sleep (~10-20% of total sleep), most difficult to awaken from. Most prominent in the first half of the night.
    • The brain is "asleep" (high arousal threshold), but the motor system is not paralyzed, allowing for complex movements. This is why parasomnias like sleepwalking (somnambulism) and night terrors occur during N3.
    • N3 sleep is considered the most physically restorative stage. During this time, the body focuses on repair and growth, such as tissue repair, bone and muscle building, and strengthening the immune system.
  • Key Associations: Parasomnias such as somnambulism (sleepwalking), night terrors, and bedwetting (enuresis) occur here.

REM (Rapid Eye Movement) Sleep

• EEG: Beta waves (high frequency, low amplitude), similar to the awake state; "sawtooth waves" are also characteristic.
• Description: "Paradoxical sleep" as the brain is highly active while the body is immobile (~20-25% of total sleep). Duration of REM periods increases with each cycle throughout the night.
  • Not the "Deepest": Although you are paralyzed, your arousal threshold during REM sleep is actually lower than in N3 sleep. It is generally easier to wake someone from REM sleep. When awakened from REM, people often feel alert and can vividly recall their dreams, unlike the grogginess ("sleep inertia") experienced when woken from N3.
• Physiology:
  • Rapid eye movements.
  • Atonia: Generalized muscle paralysis (except for extraocular muscles and diaphragm). This is protective, preventing one from acting out dreams.
  • Vivid dreaming and nightmares occur.
  • Variable heart rate and blood pressure, penile/clitoral tumescence.
• Key Associations:
  • Important for memory consolidation.
  • Depression is associated with increased total REM sleep and decreased REM latency (earlier onset of REM).
  • REM sleep behavior disorder: Loss of atonia leads to dream enactment (e.g., punching, kicking).

Parasomnias

Sleep terror disorder

• Definition: a non-REM-related parasomnia that occurs during the N3 sleep stage (slow-wave sleep), characterized by episodes of sleep terror
• Epidemiology: Discrete episodes of sleep terrors are relatively common in children (∼ 20% of children and ∼ 2% of adults), but the disorder is rare.
• Risk factors
  • Stress or fatigue
  • Fever
  • Sleep deprivation
• Clinical features
  • Screaming or crying suddenly upon awakening, usually in the first part of the night (rarely during daytime naps)
  • Intense fear and agitation
  • Tachypnea, diaphoresis, tachycardia during episodes
  • Difficulty arousing patients during episodes
  • Patients usually return to sleep after the episode.
  • Typically no recollection of the arousal episode (unlike with nightmare disorder)
    • Nightmare disorder
      • a REM-related parasomnia characterized by recurrent nightmares
      • Patient remembers the dream after awakening (REMember)

REM sleep behavior disorder

• Definition: a REM-related parasomnia characterized by dream enactment due to loss of REM sleep atonia
• Epidemiology
  • Sex: ♂ > ♀
  • Usually in older patients (> 50 years)
• Risk factors
  • Narcolepsy
  • Psychiatric medications (e.g., antidepressants)
  • Neurodegenerative disorders (e.g., Parkinson disease, Lewy body dementia)
• Clinical features
  • Physically acting out dreams during sleep (e.g., yelling, moving limbs, walking, punching), sometimes leading to injury to self or others
  • Patient is alert and orientated after awakening, and remembers the dream.
• Prognosis: Most patients with idiopathic RBD eventually develop a disorder of α-synuclein neurodegeneration, most commonly Parkinson disease.