Sleep Apnea is often caused by obesity

A blockage or narrowing of the airways in your nose, mouth, or throat generally causes obstructive sleep apnea (OSA). This usually occurs when the throat muscles and tongue relax during sleep and partially or completely block the airway. When you stop breathing or have reduced flow of air into your lungs during sleep, the amount of oxygen in your blood decreases briefly.
Obstructive sleep apnea can also occur if you have bone deformities or enlarged tissues in your nose, mouth, or throat. For example, you may have enlarged tonsils. During the day when you are awake and standing up, this may not cause problems. But when you lie down at night, the tonsils can press down on your airway, narrowing it and causing sleep apnea.
See pictures of normal and blocked airways during sleep.
Other factors that may contribute to sleep apnea include:

• Drinking alcohol, which affects the part of the brain that controls breathing. This may relax the breathing muscles and cause a narrowing of the airway and sleep apnea.
• Obesity. Fat in the neck area can press down on the tissues around the airways. This narrows the airways and can cause sleep apnea. About 70% of people who have sleep apnea are obese.
• Some medicines that are taken for conditions such as allergies, depression, insomnia, or anxiety. These medicines, especially sedatives, can also relax the muscles and tissues in the throat, causing it to narrow.