Why is one of my nose holes always close?

Rhinitis: Nasal congestion is more common when you lay down, structures inside the nose called turbinates can swell. This may be aggravated by environmental irritants or allergens in your bedroom. Additionally, you may have a deviated septum, the structure that divides your nostrils in the center of your nose internally. Mucus can indicate nasal irritation or allergy. I recommend you see an ENT to evaluate.