Sleep Architecture

Sleep architecture is the term used to describe the cyclical pattern of sleep as it progresses through different stages over the course of the night. It includes:

• Types of sleep: Non-REM (NREM) and REM sleep
• Stages within NREM: Traditionally divided into stages 1, 2, and 3 (note: recent classifications by the American Academy of Sleep Medicine [AASM] consolidate Stage 3 and 4 into one deep sleep stage, often referred to as N3 or slow-wave sleep)
• Sleep cycles: Typically repeat every 90–120 minutes through the night
• Periods of wakefulness: Both brief and sustained awakenings also form part of sleep architecture

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Stages of Sleep

1. Wakefulness
   Periods before sleep onset and during the night when the individual may briefly wake up.
2. Non-REM Sleep
   • Stage 1 (N1): Light sleep, transition from wakefulness. Easily disturbed.
   • Stage 2 (N2): Onset of true sleep, body temperature drops, and heart rate slows.
   • Stage 3 (N3, formerly Stage 3 and 4): Deep (slow-wave) sleep. Important for physical restoration, immune function, and growth hormone release.
3. REM Sleep
   • Characterized by rapid eye movements, muscle atonia, and vivid dreaming.
   • Important for cognitive functions such as memory consolidation and emotional regulation.

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Sleep Duration vs. Sleep Quality

• Sleep Duration: The total number of hours a person sleeps in 24 hours, comprising both NREM and REM sleep.
• Sleep Quality: Affected not just by duration, but also:
  • Sleep onset latency (how quickly one falls asleep)
  • Sleep continuity (how often one wakes during the night)
  • Time spent in restorative stages (especially N3 and REM)

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Clinical Significance

Alterations in sleep architecture can occur with:

• Ageing (e.g., less slow-wave sleep)
• Medical and psychiatric conditions (e.g., insomnia, depression, OSA)
• Medications (e.g., benzodiazepines suppress REM, increase N2)
• Substance use (e.g., alcohol fragments REM later in the night)

Disrupted or altered sleep architecture can lead to:

• Daytime sleepiness
• Impaired memory and concentration
• Poor cardiovascular and metabolic health