During sleep, air should move freely and rhythmically in and out of the lungs via the nose and/or mouth just as while awake. When this normal breathing pattern is disrupted during sleep, "sleep-disordered breathing" is said to occur. Sleep-disordered breathing refers to a range of breathing disturbances, including apneas, hypopneas, snoring, and respiratory effort-related arousals.
When airflow completely stops during sleep, it's called an apnea. When airflow slows significantly, but not completely, it's called an hypopnea. In obstructive sleep apnea (OSA), apneas and hypopneas occur because of complete or near-complete blockage of the upper airway (throat). In central sleep apnea (CSA), apneas and hypopneas occur because the brain and central nervous system fails to generate a rhythmic signal to breathe. OSA is much more common than CSA. CSA typically occurs only in people who have experienced heart failure, stroke, or kidney failure.
Snoring occurs when air flows through a narrowed throat. The turbulence created by the air as it passes relaxed tissues causes them to vibrate, and that vibration is the sound of snoring. Because snoring and OSA are both due to narrowing of the throat, people who snore are more likely to have OSA.
Sometimes the throat narrows enough to cause loud snoring and hard breathing, but not enough to produce apneas or hypopneas. Breathing in this situation can be difficult enough to wake the sleeper, an occurrance referred to as a respiratory effort related arousal (RERA). Most people with OSA have some combination of loud snoring, RERAs, hypopneas, and apneas when they sleep, so the term OSA is commonly used to encompass all of these phenomena.
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