Awake Airway Patency
Wakefulness keeps pharyngeal dilator muscles firing and the airway open.
- High: Genioglossus activity (tongue held forward)
- 100%: Airway cross-section (fully patent baseline)
- Normal: Airflow (unobstructed breathing)
- Very low: Collapsibility (Pcrit) (stiff, resistant airway)
A muscular tube, not a rigid pipe
The pharynx has no bony support, unlike the trachea.
Dilator muscles hold it open
Genioglossus and palate muscles actively brace the walls awake.
Neural drive from the brainstem
Wakefulness stimulus keeps motor neurons continuously active.
Sleep Onset & Falling Muscle Tone
At sleep onset, wakefulness drive fades and dilator tone starts dropping.
- Falling: Genioglossus activity (losing wake drive)
- ~80%: Airway cross-section (mild narrowing)
- Slightly reduced: Airflow (increased resistance)
- ~10–20 min: Onset latency (typical for adults)
The wakefulness stimulus withdraws
Losing this drive is the single biggest tone change.
Airway becomes more compliant
Softer tissue starts to sag inward with each breath.
Snoring may begin here
Vibrating soft tissue produces the first audible sign.
REM and Deep Sleep Muscle Atonia
REM sleep brings near-total skeletal muscle atonia, narrowing the airway further.
- Minimal: Genioglossus activity (near-atonic in REM)
- ~40%: Airway cross-section (markedly narrowed)
- Reduced: Airflow (audible turbulence)
- High suction: Negative pressure (inspiration pulls walls in)
REM atonia spares little
Motor inhibition during REM extends to airway dilators.
Bernoulli effect accelerates collapse
Fast airflow through a narrow gap drops local pressure.
Body position matters
Supine posture lets gravity pull the tongue backward.
Soft Palate & Tongue Base Collapse
The soft palate and tongue base meet the pharyngeal wall, sealing the airway.
- ~0%: Airway cross-section (complete occlusion)
- 0 L/min: Airflow (apnea in progress)
- Falling: Blood oxygen (desaturation begins)
- 10–40 sec: Event duration (typical apnea length)
Two collapse sites dominate
Retropalatal and retroglossal levels close in sequence.
Respiratory effort continues uselessly
Chest and diaphragm keep straining against a sealed airway.
Oxygen and carbon dioxide drift
Falling oxygen and rising carbon dioxide trigger chemoreceptors.
Arousal Restores the Airway
Chemoreceptor alarms trigger a brief arousal, restoring tone and reopening the airway.
- Chemoreflex: Arousal trigger (low O2 / high CO2)
- Surging: Genioglossus activity (reflex reactivation)
- Restored: Airway cross-section (gasp reopens passage)
- Nightly: Cycle repeats (often unremembered)
A protective micro-awakening
The brainstem briefly lightens sleep to restore muscle tone.
A gasp or snort reopens flow
Sudden dilator activation snaps the airway back open.
Sleep fragments across the night
Repeated arousals prevent restorative deep and REM sleep.