Sleep apnea happens when something repeatedly blocks or disrupts your breathing while you sleep. In the most common form, the soft tissues in your throat physically collapse inward. In a less common form, your brain temporarily stops sending the signal to breathe. About 15 to 30 percent of middle-aged men and 10 to 15 percent of middle-aged women are affected, though many don’t know it.
The causes range from the shape of your face and jaw to your weight, age, hormones, and even what you drank before bed. Understanding why it happens starts with what’s going on inside your airway every time you fall asleep.
What Happens in Your Throat During Sleep
Your upper airway is essentially a flexible tube made of soft tissue and muscle. When you’re awake, muscles in your tongue, soft palate, and throat walls actively hold this tube open. The most important of these is a fan-shaped muscle at the base of your tongue called the genioglossus. It contracts just before each breath, pulling your tongue forward and keeping the airway clear.
When you fall asleep, the brain dials down these muscle signals. The tongue relaxes backward, the soft palate sags, and the walls of the throat narrow. In most people, the airway stays open enough to breathe normally. But if the airway is already narrow for any reason, this relaxation can tip the balance. As the opening shrinks, air speeds up through the tighter space, which actually lowers the pressure inside the airway (the same physics that keeps an airplane wing aloft). That drop in pressure pulls the walls inward even more, and the airway collapses.
This is worst during REM sleep, the dreaming stage. REM triggers a near-total shutdown of skeletal muscle activity throughout the body, including the tongue and throat muscles. Breathing events during REM tend to last longer and cause deeper drops in blood oxygen than those in other sleep stages.
Excess Weight and Neck Size
Obesity is the single strongest modifiable risk factor. Fat doesn’t just accumulate under your skin. It deposits around the tongue, along the walls of the pharynx, and in the soft palate, physically narrowing the airway from the outside. Even modest weight gain can shift someone from snoring into full airway obstruction.
Neck circumference is a practical marker of this risk. A neck larger than 17 inches in men or 16 inches in women signals excess fat in the neck area and a meaningfully higher chance of obstructive sleep apnea. But neck size is just one piece. Fat distribution matters as much as total body weight, which is why some people with a normal BMI still develop sleep apnea if they carry fat disproportionately around the throat and tongue.
Jaw Shape and Facial Structure
Some people are born with airways that are set up for trouble. A small or recessed lower jaw (retrognathia) pushes the base of the tongue closer to the back of the throat, leaving less room for air. A narrow upper jaw, an elongated soft palate, a large tongue, or a hyoid bone that sits lower than usual all contribute to a tighter airway. Research consistently shows that people with sleep apnea tend to have a combination of these features: a smaller, more set-back mandible, a narrower posterior airway space, and enlarged soft tissue structures.
These traits are largely inherited. Studies using MRI imaging have found that the volume of the tongue, the lateral pharyngeal walls, and total soft tissue around the airway are 35 to 40 percent heritable. Specific craniofacial dimensions show similar patterns: maxillary (upper jaw) width is about 47 percent heritable, mandibular width about 30 percent, and the size of the space behind the mouth and throat about 31 percent. This is why sleep apnea often clusters in families, even among members who aren’t overweight.
Age, Sex, and Hormones
Sleep apnea becomes dramatically more common with age. Among women under 50 with a normal BMI, prevalence is roughly 3 percent. After 50, it jumps to about 16 percent. For men with a normal BMI, it rises from around 11 percent before age 50 to nearly 38 percent after. Aging brings loss of muscle tone in the throat, changes in fat distribution, and shifts in how the brain controls breathing during sleep.
The gap between men and women narrows significantly after menopause. Before menopause, estrogen and progesterone appear to help maintain airway muscle tone and influence where the body stores fat. When those hormones decline, women’s risk climbs toward men’s rates. Testosterone, meanwhile, may promote fat deposition around the upper airway and alter ventilatory control, which partly explains why men are affected more often throughout adulthood.
Alcohol, Sedatives, and Sleeping Position
Alcohol relaxes the muscles of the mouth and throat beyond what normal sleep does, making it significantly easier for loose tissue to block the airway. It also slows the central nervous system in a way that raises your arousal threshold. Normally, when your oxygen drops during an apnea event, your brain partially wakes you just enough to restore muscle tone and reopen the airway. Alcohol makes it harder for that rescue signal to get through, so apnea events last longer and oxygen levels fall further.
Sedative medications work through a similar mechanism. Any drug that depresses the central nervous system, including certain sleep aids and anti-anxiety medications, can worsen both obstructive and central sleep apnea by reducing muscle tone and blunting the brain’s response to low oxygen.
Sleeping on your back allows gravity to pull the tongue and soft palate directly backward into the airway. Many people experience apnea events exclusively or predominantly in the supine position, which is why side sleeping is one of the simplest interventions.
When the Brain Forgets to Breathe
Central sleep apnea is a different problem entirely. Instead of a physical blockage, the brain’s respiratory control center temporarily stops sending the signal to breathe. A cluster of neurons in the brainstem normally generates the rhythmic drive that tells your diaphragm and chest muscles to contract. In central sleep apnea, this rhythm falters, and you simply skip one or more breathing cycles.
This form of apnea is most commonly linked to heart failure, which disrupts the chemical feedback loops the brain uses to regulate breathing. It also occurs with damage to the brainstem from stroke, kidney failure, and opioid use. In newborns, central apnea can result from an immature respiratory system that hasn’t fully developed its automatic breathing rhythm.
Children and Sleep Apnea
In children, the primary cause is different from adults. Enlarged tonsils and adenoids are by far the most common reason, especially in younger kids. These lymphoid tissues sit right at the back of the throat and nasal passage, and when they’re swollen, they can physically block the airway during sleep. Unlike adult sleep apnea, which typically requires long-term management, pediatric cases caused by enlarged tonsils and adenoids often resolve after surgical removal.
Childhood obesity is an increasingly important factor as well, mirroring the same airway-narrowing mechanics seen in adults. Children with certain craniofacial conditions or neuromuscular disorders that affect throat muscle tone are also at higher risk.
Medical Conditions That Raise Risk
An underactive thyroid can contribute to sleep apnea through multiple pathways. Hypothyroidism causes a type of muscle weakness that can affect the upper airway muscles, reducing their ability to keep the throat open. It also promotes weight gain and fluid retention, both of which narrow the airway. Notably, treating the thyroid condition alone doesn’t always resolve the apnea. In one clinical series, six out of eight patients had persistent sleep apnea even after their thyroid levels returned to normal, suggesting that once the airway changes are established, they can become self-sustaining.
Conditions involving excess growth hormone can enlarge the tongue and soft tissues of the throat, creating obstruction. Nasal congestion from allergies or structural issues like a deviated septum forces mouth breathing, which changes airway dynamics and increases the chance of collapse. Even fluid retention from conditions unrelated to weight, such as kidney disease, can cause fluid to shift into the neck tissues when you lie down, narrowing the airway overnight.
Why Multiple Factors Usually Overlap
Sleep apnea rarely comes down to a single cause. A person with a mildly recessed jaw might breathe fine at age 30 but develop apnea at 55 after gaining 20 pounds and losing some muscle tone with age. Someone with a normal weight but a naturally narrow airway might only have events during REM sleep or after drinking alcohol. The condition sits at the intersection of anatomy, body composition, neurological control, and behavior, which is why it shows up in such a wide range of people and why it often worsens gradually rather than appearing overnight.