Sleep

Describe the physiology of sleep

Sleep is a naturally occurring state of unconsciousness from which one can be aroused by an external stimulus.

Sleep is important in:

• Homeostasis of many organ systems
• Memory formation
• Preservation of cognitive function

Stages of Sleep

Stages of sleep are classified based on EEG changes:

• REM sleep
  Characterised by EEG activity resembling that of awake individuals. REM sleep:
  • Lasts for 5-30 minutes
    Event frequency decreases with age.
  • In REM sleep:
    • Irregular eye movements
    • Dreaming occurs
    • Irregular HR and RR
    • Muscle contraction occurs (but muscle tone is decreased)

• Non-REM sleep
  Deep sleep, characterised by depression of HR, SVR, BP, RR, and metabolic rate (~0.9 METs) It is divided into four stages on EEG:
  • Stage 1: 4-6Hz θ waves replace α-waves
    Dosing, easily roused.
  • Stage 2: Similar to stage 1 with occasional high frequency 50μV bursts (sleep spindles)
  • Stage 3: 1-2Hz high-voltage δ waves appear
  • Stage 4: Large δ waves become synchronised
    Deep sleep.

Periods of REM sleep alternate with non-REM sleep during the night, with an average of 4-5 cycles of REM sleep per night.

Respiratory Effects

GABAergic neurons depress the respiratory centre, leading to respiratory depression:

• Decreased MV
  • Decreased V_T
    Greatest decrease occurs during REM sleep, where it falls by ~25%.
  • Unchanged RR
• Increased PaCO₂
• Decreased PO₂
  More pronounced in elderly.

• Collapse of airway soft tissue
  Due to reduced tonic activity of pharyngeal muscles.

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.
3. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.