Why do people with asthma cough more at night?

My doctor told me that is a symptom of asthma when I had this problem in April with a mild case of bronchitis. I currently have an upper respiratory infection and even though I don't cough too much during the day, I take cough syrup and my antibiotics as prescribed, within a couple of minutes of laying down, I get this TERRIBLE cough. I am not even really coughing anything up anymore. It sounds like a barking seal! I am going to ask him if I need to be tested, but while I am incredibly tired but awake at 1 AM, I thought I'd ask the Yahoo Answers community! :-)