I experience nasal stuffiness during the day due to tree pollen allergies,but symptoms get worse at night (when pollen count is lower). any ideas why?

Position/activity: During the day, you are more active. you are up, around, doing things. At night, especially when you are in bed, you are less active and lying supine (on your back). This decrease in activity and your position in bed may both contribute to worse congestion at night. The other thing that I would consider is that your bed may be filled with allergens. Wash your bedclothes and see what happens.