Obstructive sleep apnea happens when the soft tissues in your throat repeatedly collapse during sleep, blocking airflow for seconds at a time. An estimated 83.7 million American adults live with the condition, roughly 32% of everyone over age 20. The causes range from anatomy you’re born with to weight changes, hormonal conditions, and even the position of fluid in your body while you sleep.
How the Airway Collapses During Sleep
Your upper airway stays open during the day because muscles in the throat actively hold it open. The most important of these is the genioglossus, the fan-shaped muscle that anchors your tongue to your lower jaw and pulls it forward, keeping it out of the airway. A second set of muscles runs along the front of the neck and works to widen and stiffen the passage.
When you fall asleep, the brain dials down activity in these airway muscles more than it reduces the effort of the diaphragm. The diaphragm keeps pulling air inward, but the throat muscles are no longer holding the passage open firmly enough to resist that suction. The result is a partial or complete collapse, most often at the narrowest point: the space directly behind the soft palate. About half of people with obstructive sleep apnea have their primary obstruction above the uvula, while the other half collapse lower in the throat, in the area behind the base of the tongue.
Each collapse triggers a brief arousal, just enough for the brain to reactivate the throat muscles and reopen the airway. The cycle can repeat dozens of times per hour. Severity is measured by the apnea-hypopnea index (AHI), which counts how many times breathing drops or stops per hour of sleep. An AHI between 5 and 15 is mild, 15 to 30 is moderate, and 30 or higher is severe.
Excess Weight and Neck Fat
Obesity is the single strongest modifiable risk factor. It works through several pathways at once. Fat deposits can accumulate inside the walls of the pharynx and within the airway muscles themselves, physically narrowing the passage and weakening the muscles’ ability to hold it open. A large abdomen also pushes the diaphragm upward, reducing the downward tug on the trachea that normally helps keep the throat taut.
Neck circumference is a practical screening marker. A neck larger than 17 inches in men or 16 inches in women is considered a significant risk factor, according to Mayo Clinic guidance. That measurement captures both the subcutaneous fat around the neck and the deeper deposits pressing on the airway from the sides.
Jaw and Facial Structure
Some people develop sleep apnea at a normal weight because their skeletal anatomy leaves less room for the airway in the first place. A recessed lower jaw (retrognathia) positions the tongue base closer to the back wall of the throat, making collapse easier. This pattern shows up even in young children: preschoolers with mild obstructive sleep apnea tend to have a measurably smaller, more set-back mandible compared to children without the condition.
Other structural features that reduce airway space include a naturally large tongue, a thick or low-hanging soft palate, and a high, narrow hard palate. These traits are partly genetic, which is why obstructive sleep apnea often clusters in families. You can also develop structural crowding later in life from conditions like acromegaly, where excess growth hormone causes the tongue and soft tissues to enlarge.
Fluid Shift During Sleep
One of the less obvious causes involves gravity and fluid redistribution. During the day, fluid pools in your legs. When you lie down at night, that fluid migrates upward toward the neck, a phenomenon researchers call nocturnal rostral fluid shift. Studies published in Circulation showed that this displacement measurably increases neck circumference overnight, narrows the pharynx, and makes it more collapsible.
The relationship is surprisingly powerful. Airway resistance increases to the fourth power of any reduction in the pharynx’s radius, so even a small amount of extra fluid in the neck tissue can dramatically raise the odds of obstruction. This mechanism is especially relevant for people with heart failure or chronic venous insufficiency, where fluid retention in the legs is already pronounced. It also helps explain why sleeping with your head elevated or using compression stockings during the day can reduce apnea severity in some people.
Alcohol, Sedatives, and Muscle Relaxants
Alcohol increases airway resistance in both the nose and throat, even in people without a sleep apnea diagnosis. It relaxes the same upper airway muscles that are already vulnerable during sleep, tipping the balance further toward collapse. Drinking within a few hours of bedtime can turn mild, subclinical apnea into something noticeable, or make existing moderate apnea significantly worse for that night.
Sedative medications, particularly benzodiazepines, work through the same brain receptor system (GABA) that controls upper airway muscle tone. Research from the Anesthesia Patient Safety Foundation shows that increasing doses of sedatives reduce activity in the airway nerves and muscles more profoundly than they suppress the diaphragm. This creates the exact mismatch that causes obstruction: the diaphragm keeps generating suction while the throat muscles go slack.
Hormonal and Endocrine Conditions
Hypothyroidism can cause the pharyngeal walls to thicken due to a buildup of certain proteins and fluid in the tissue, a process that narrows the airway from the inside. In some cases, treating the thyroid condition with hormone replacement reverses the airway changes enough to resolve the apnea entirely. Acromegaly works through a similar mechanism: excess growth hormone enlarges the tongue and thickens the soft tissue lining the throat.
Hormones also influence breathing drive at the level of the brain and the chemical sensors that detect oxygen and carbon dioxide. This is one reason obstructive sleep apnea is far more common in men before age 50, but the gap narrows after menopause, when estrogen and progesterone levels drop. Both hormones appear to have a protective effect on upper airway muscle tone and breathing regulation.
Why Children Get Sleep Apnea
In adults, obesity is the dominant driver. In children, the primary cause is different: enlarged tonsils and adenoids. The tonsils sit at the back of the mouth while the adenoids occupy the back of the nasal passage, and when both are swollen, they can physically block the airway during sleep. This is why tonsillectomy and adenoidectomy are the first-line treatment for most children with obstructive sleep apnea, and why the condition often resolves after the procedure.
Childhood obesity is an increasingly relevant factor, but even among overweight children, large tonsils and adenoids remain the strongest predictor, especially in younger kids. Craniofacial differences, such as a small jaw or a narrow midface, also contribute in a subset of pediatric cases, particularly in children with Down syndrome or other conditions that affect facial bone development.
Multiple Causes Often Overlap
Most people with obstructive sleep apnea don’t have a single cause. A moderately recessed jaw might never cause problems on its own, but combine it with 20 extra pounds, a nightly glass of wine, and the normal fluid shift that comes with aging, and the airway reaches a tipping point. This layering effect explains why sleep apnea often appears in middle age even when anatomy hasn’t changed. It also explains why weight loss of 10 to 15% of body weight can sometimes cut AHI in half, not because weight was the sole cause, but because it was the factor that pushed a borderline airway over the edge.
Of the estimated 83.7 million American adults with obstructive sleep apnea, about 52% have mild disease, 30% moderate, and 18% severe. Severity tends to correlate with how many contributing factors are present and how extreme each one is. Recognizing which causes apply to you is useful because some, like weight, alcohol use, and sleeping position, are modifiable, while others, like jaw structure, may require mechanical or surgical solutions.