coughing, but why only in the evening?

[Doc]

Start with asthma and allergies or irritation from smoking.

Temps drop. The air hold less moisture.

The heat comes on. Dries things out more.

Bronchial passages dry out.

Coughing produces phlegm and gets mucus moving, lubricating, moisturizing and cleaning respiratory surfaces.

...

Take a shower at night and keep a humidifier by your bed.

Edit: I'm not a medical doctor. But I've been through the same sh!t with coughing at night. If it gets really bad, I boil some water and suck the vapor for awhile.