A sneeze begins with a tickling sensation in the nerve endings that sends a message to your brain that it needs to rid itself of something irritating the lining of your nose. Sneezing is a procedure of expelling the harmful bacteria and virus which has entered the body and multiply themselves during cold, allergic reactions or when dust blows on our face.The high velocity of the airflow is achieved by the buildup of pressure inside the chest with the vocal chords closed. Sudden opening of the cords allows the pressurized air to flow back up the respiratory tract to expel the irritants. This helps to remove offending particles in the nose. It is an important reflex action to remove an irritant and to keep our air passages open and free of obstructions.
People don’t sneeze when they are asleep because the nerves involved in nerve reflex are also resting.