J.J.
Mucous from anything, be it cold or allergies or whatever always gets worse at night due to laying still and the mucous collects. That being said, humidifiers can actually make this worse in many people. Both my children have issues with humidified air. My son can't have any on or he coughs like you described. Without the humidifier he does much better. My daughter does this with a warm mist humidifier but cold mist seems to help her in some situations. So when she needs it on I have to move one of them into bed to with me as they share a room. My son will even start coughing from it if he is not even sick. I would try stopping the humidifier. What might be better is to give her a warm shower before bed. This helps my son, but like I said he can't take any humidifier. You might find she does better without the moist air.