The most common cause is often lack of humidity, which may also lead to mouth-breathing. If you have a persistent stuffy or runny nose before bedtime or after a few hours of sleep, you should seek assistance from your health care professional.
The first step in treating this problem is ensuring you use a CPAP heated humidifier and properly clean all equipment.
If your nose is usually stuffy or runny (e.g. because of allergies), ask your doctor about nasal irrigants or prescription nasal sprays. These nasal treatments can safely be used long-term to help keep your nasal passages clear, make breathing with CPAP more comfortable and prevent mouth-breathing. Non-prescription nasal sprays and ointments containing petroleum should be avoided.