Sleep physiology / breathing
Breathing during sleep, breath by breath.
All night, one loop keeps you breathing: the throat stays open, the chest and belly pull air in, the blood carries oxygen out and carbon dioxide back, and the brain sets the next breath. Pick who is sleeping, add a risk factor or a treatment, and watch each part of the loop answer.
23:30 · N2, light sleep · on the back
Asleep, the throat muscles relax but still hold the airway open. Carbon dioxide sets the breathing, and it is steady.
Who is sleeping
The airway stays open and the breathing stays steady through every stage of sleep.
Risk factors
Things that make the airway close more easily, or change how quickly the sleeper wakes.
The night
Real time. The night view shows all of it. Sleep stages come from a normal night for this age.
Treatment
Each one works on the part of the loop it really works on: the airway, the muscles, or the breathing control.
A machine that breathes in the gaps (adaptive servo ventilation) can steady central apnea. It is not used in heart failure with an ejection fraction of 45% or less: in a large trial it raised the death rate even though it controlled the pauses (Cowie et al., N Engl J Med 2015).
For clinicians: the loop's settings
Add a second condition
Its changes are laid over the first; a shared setting takes the second value.
Airway collapsibility: 0.10
Lung oxygen store: normal
Muscle responsiveness: 1x
Dilator tone: 1x
Arousal threshold: normal
Controller gain: 1x
Circulation delay: 7 s
Central drive: 1x
Rhythm irregularity: regular
Inspired oxygen: room air
Nasal resistance: normal
Steps, not dials: each is low, normal or high for the model. Risk factors and treatments act on top of these.
A model, not a recording, and not anyone's own numbers. One loop runs the whole night: the throat can close when its muscles relax, the brain sets each breath from the carbon dioxide it senses a few seconds late, oxygen travels from the lungs to the finger, and a brief arousal can open the throat again. Nothing about an event is scripted. The four weak points in the loop come from a study of people with sleep apnea: 36% had throat muscles that answered poorly, 37% woke too easily and 36% had a jumpy breathing control (Eckert et al., Am J Respir Crit Care Med 2013). In heart failure, one cycle of waxing and waning breathing takes about 59 seconds, against 37 seconds with a normal heart, because the blood takes longer to get around (Hall et al., Am J Respir Crit Care Med 1996). At high altitude the cycles take about 23 to 30 seconds (Lombardi et al., J Sleep Res 2013). The painting was made with an image tool and its anatomy checked by eye.
What to try
Start with Normal and press Cause an apnea. The throat shuts, the chest keeps pulling while the belly pushes out, and the oxygen number in the corner starts to fall a few seconds later, because the blood takes time to reach the finger. Then the brain wakes for a moment, the throat snaps open, and breathing catches up. Nobody remembers these brief wake ups in the morning.
Now choose Sleep apnea. The same thing happens by itself, again and again, worse on the back and in dream sleep. Turn on CPAP and watch the air pressure hold the throat open, even though the muscles are just as relaxed as before. Turn the pressure down and the narrowing comes back.
Heart failure breathing and high altitude look different: the throat stays open, and the pauses come from the brain. The loop view shows why. The brain reads the carbon dioxide in the blood a few seconds late and answers too hard, so each correction overshoots the last.
Note: This is a simulation built to explain how breathing works in sleep. Every trace and every oxygen number here comes from a model. None of it is a recording, and nothing on this page diagnoses anything. Sleep apnea is diagnosed with a sleep study. See the article on obstructive sleep apnea for what the condition is and how it is checked.
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Not for pages that sell a sleep product or run advertising around it. If you use it, an email to matthew@bettersleepproject.com is welcome but not required.