K1vii / 15B18: Describe the stages of sleep

15B18: Exam Report

Describe the stages of sleep (50% of marks). Describe the respiratory physiological changes that occur in sleep (50% of marks).

29% of candidates passed this question.

Few candidates demonstrated a good knowledge of this topic. Few answers described the EEG changes associated with the stages of sleep. Respiratory changes in sleep were more commonly known though many candidates made no reference to the change in resistance associated with reduction in upper airway tone.

Confusion existed about the tidal volume changes in sleep. The question asked specifically for respiratory changes and marks were not awarded for discussion about cardiovascular or metabolic responses.

K1vii / 14A01: Describe the stages of sleep


Sleep is necessary reversible ↓consciousness from which one can be roused by sensory stimuli

Stages of Sleep

  • Sleep is cyclical
  • 2 phases
  • Sleep cycles between these 2 states every 90’
  • ∴~3 – 6 x /night
  • At beginning of night 80’ NREM + 10’ REM
  • As night progresses, ↓NREM stage 3 + 4 & ↑REM

Non-Rem Sleep

  • ‘Inactive brain, moveable body’
  • 4 stages: ↑depth of sleep & ↑arousal thresholds

Stage 1: 5%

  • Easy arousal
  • α & β waves
  • Hypnotic jerks

Stage 2: 50%

  • Sleep spindles
  • K complexes
  • No consciousness of surroundings

Stage 3 & 4

  • δ waves (high amp, low frequency)
  • Difficult to wake

REM Sleep

  • ‘Highly active brain, paralysed body’
  • Rapid eye movements
  • Muscle atonia
  • EEG = β waves →resembles wakefulness

Physiological ∆ of Sleep


NREM Sleep

↑parasymp tone

Non vivid, non-memorable dreams


REM Sleep

↑parasymp tone

Vivid dreams

CMRO2 = awake


NREM Sleep




REM Sleep




NREM Sleep

  • ↓FRC (supine + ↓muscle tone)
  • Dependent areas of atelectasis
  • ↓response ↑PaCO2
  • ↓SpO2
  • ↓O2 consumption
  • ↑Tv + ↓RR →↓MV progressively with stages

REM Sleep

  • ↓FRC (supine + ↓muscle tone)
  • Dependent areas of atelectasis
  • ↓response ↑PaCO2
  • Collapsed upper airway m’s →OSA
  • ↑RR + ↓VT →↓MV muscle atonia
  • Resp activity due to diaphragm only

Skeletal m

NREM Sleep

↓muscle tone progressively with stages

REM Sleep

↓↓↓ muscle tone






↑Motility (PNS tone)


GH release in beginning

@ midnight = cortisol ???

Then cortisol progressively ↑, peaks @ 0900hr

60 min after wakening ↑↑↑↑ cortisol + ACTH secretion



↓Temp (loss of thermoregulation in REM)

EEG – Electroencephalography

  • Recording of electrical activity of the brain
    • 20 reference electrodes
    • Anatomically placed
    • 20 lines graphed
    • Sum of thousands of post-synaptic potentials generate an electric current →recorded at the surface
    • Represents the voltage difference b/w 2 electrode locations
    • 3 parameters recorded:
      1. AMPL – electrical height
      2. FREQ – no. times per sec wave touches zero voltage line
      3. TIME
    • 4 waves
      1. β > 13Hz →awake, excited, alert, REM
      2. α 8 – 13Hz →awake, relaxed, quiet rest
      3. θ 4 – 8Hz →sleeping
      4. δ 0 – 4Hz →deep sleep, encephalopathy
    • EEG →records the electrical activity of millions of neurons →NON-SPECIFIC

Evoked Potentials: triggered →very specific

  • Evoked potentials = polyphasic waves produced after stimulation of specific neural tracts
  • They can detect slowed electrical conduction e. due to demyelination along pathways even when there is no clinical sign yet
  • e. MS diagnosis requires 2 distinct areas of demyelination ∴EP testing can confirm diagnosis