Then the body tries to expel this mucus build up through cough in order to get relief from the irritation. This kind of inflammation in the respiratory passage can occur due to many different reasons. Some of them are: the common cold, post nasal drip, asthma, exposure to allergens like dust and smoke, smoking and so on. A unique characteristic of a cough is that it becomes severe when you lie down. This is because the mucus drains to the back of the throat which induces a cough.