Question 18

Describe the stages of sleep (50% of marks).

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

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College Answer

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.


Considering that even grown-ass intensivists are usually unable to interpret and report polysomnography data, it seems unreasonable to expect their juniormost trainees to need to know the EEG criteria for different sleep stages. One can only assume that, in order to pass, the trainees were expected to regurgitate the names of memorised EEG patterns, which they cannot possibly be expected to ever recognise.

Stages of sleep:

Stage  Observations EEG changes
NREM N1 lightest stage, easiest to rouse
  • loss of alpha rhythm
  • Presence of theta waves (characteristic frequency of 4–7 Hz).
NREM N2 50% of normal sleep
  • Spindles (burst-like trains of waves in the 11- to 16-Hz range with a total duration ≥0.5 seconds)
  • K-complexes (well-defined biphasic waves lasting ≥0.5 seconds and usually maximal over the frontal cortex)
NREM N3 difficult to rouse
  • Delta-waves, large (≥75 µV) and slow (0.5–3 Hz)
REM Characterised by rapid phasic eye movements and minimal muscle tone
  • EEG resynchronization and flattening
  • Sawtooth EEG wave morphology
  • Theta waves

Respiratory physiological changes during sleep:

Physiological Changes During Normal Sleep
Parameters NREM sleep REM sleep
Respiratory changes
Respiratory rate Decreases Variable, with apnoeas
Minute volume Decreases Decreases further
Alveolar ventilation Decreases Decreases further
PaCO2 Increases slightly Increases more
PaO2 Decreases slightly Decreases more
Hypoxic ventilatory response Decreases Decreases further
Hypercapnic ventilatory response Decreases Decreases further
Upper airway muscle tone Decreases slightly Decreases further
Upper airway resistance Increases Increases further


Orem, John, ed. Physiology in sleep. Elsevier, 2012.  - via Googlebooks

Carley, David W., and Sarah S. Farabi. "Physiology of sleep." Diabetes Spectrum 29.1 (2016): 5-9.

Chokroverty, Sudhansu. "Physiological changes of sleep." Sleep Disorders Medicine. Springer, New York, NY, 2017. 153-194.

Carskadon, Mary A., and William C. Dement. "Normal human sleep: an overview." Principles and practice of sleep medicine 4.1 (2005): 13-23.

Bonnet, M. H., and D. L. Arand. "24-Hour metabolic rate in insomniacs and matched normal sleepers." SLEEP-NEW YORK- 18 (1995): 581-581.

Chen, Hsiun-Ing, and Ya-Ru Tang. "Sleep loss impairs inspiratory muscle endurance." American Review of Respiratory Disease 140.4 (1989): 907-909.

White, David P., et al. "Sleep Deprivation and the Control of Ventilation 1–3." American Review of Respiratory Disease 128.6 (1983): 984-986.

Orzeł-Gryglewska, Jolanta. "Consequences of sleep deprivation." International journal of occupational medicine and environmental health 23.1 (2010): 95-114.