What is Sleep?

Sleep can be defined as a physiologic state of perceptual disengagement from and unresponsiveness to the environment characterized by two distinct states – NREM (Stage N) and REM (Stage R) sleep.

Sleep and Aging

With increasing age, we see the following changes to sleep physiology:

  • Reduced total sleep time.
  • Reduced slow wave sleep (SWS).
  • No change in proportion REM sleep.
  • More arousals.
  • Increased periodic leg movements.
  • Increased number and incidence of sleep-related breathing difficulties.

Arousal Response

Arousal responses to respiratory stimuli such as hypoxia and hypercapnea are innate safeguards that protect the body from asphyxiation. Upper airway receptors provide a potent arousal stimulus, especially from REM sleep.  


Apnea can be defined as a 10 second or greater reduction of airflow of 90% or less from baseline.


Hypopnea is a 10 second or greater reduction of more than 50% in airflow from baseline with at least a 3% desaturation or an arousal.

Sleep Apnea

Sleep apnea is determined from data collected through sleep diagnostic testing called polysomnography and the degree of severity is calculated by the Apnea Hypopnea Index (AHI). AHI = Number of Apneas + Hypopneas ÷ Total Sleep Time

Sleep apnea Severity

Mild sleep apnea:  5-15 events/hr

Moderate sleep apnea:  5-30 events/hr

Severe sleep apnea:  > 30 events/hr

Classification of Sleep Apnea

Obstructive – Occurs when airflow ceases or decreases during breathing while sleeping due to the airway being narrowed or blocked.

Central – Occurs when breathing during sleep repeatedly stops due to an interruption in the signal from the brain to the muscles that control breathing.

Mixed – Presents with a combination of obstructive and central sleep apneas.

* The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications. Westchester, Ill: American Academy of Sleep Medicine; 2007.