As you fall asleep, your tongue and throat muscles relax. This allows the soft tissue at the back of your throat, including your soft palate and uvula, to sag and vibrate as you breathe and air enters and leaves your airway. The vibrations and the struggle of the air to move through the narrowed passage results in the sounds of snoring. Mild snoring by itself is usually not a medical problem, although there is increasing evidence that the vibrations can cause carotid artery vessel damage.
As the airway continues to narrow and becomes more and more obstructed, Obstructive Hypopnea events start to occur. They are defined by a reduction in airflow of over 50% of baseline with a 3% O2 desaturation, OR a reduction in airflow of over 30% with a 4% O2 desaturation AND lasting for at least 10 seconds.