A problem in the brain's respiration center

Malfunction in the serotonin system

When you sleep all of your body's muscles relax more than they do during waking hours. This doesn't cause problems for most people, but in some people with small throats this relaxation lets the airway narrow and interferes with breathing. Sleep then becomes a time of increased health risk. A smaller-than-normal jaw, large tongue, enlarged tonsils, or tissues that partially block the entrance to the airway can be factors. Sometimes several of these conditions are present in the same person.

Alcohol, sleeping pills, and tranquilizers taken at bedtime also reduce muscle tone and can make the throat more likely to collapse. Some people with OSAS may actually sleep worse when they take a sleeping pill.

While most people with OSAS have no apparent physical flaw that interferes with their breathing during sleep, some physical conditions can play a role. 
OSAS most often strikes overweight men. A different throat structure may protect women. In later years, the gap between the sexes narrows, although it never disappears completely.