What Causes Excessive Mucus In Throat?

I have a lot of mucus in my throat at night and sometimes wake up and cannot inhale or exhale, throat is clogged or closed....gasping to get air in or out...normally takes several minutes to try and get air through. Best way to describe what I go though is like putting a straw in your mouth and putting your finger at the end and put a pin hole in it and than try to breath in and out. Can take several hours to finally feel normal.