Use a humidifier. Use a humidifier to add moisture to the air, particularly during winter months. Allergens may be causing nasal mucosa irritation. Consider using anti-allergenic pillow and mattress covers. Try over the counter decongestants for a few days, if you are over 6. If no improvement, see your physician or ENT to rule out sinusitis, a nasal septal deviation, polyp or other structural abnormality.
Several approaches. Nasal saline rinses with a neti pot are a simple way to try to keep your nasal passages clear. It is important to use a saline (mild salt water) solution when you do this. Never use plain tap water. Also breathe right strips can help open your nasal passages. Consider seeing a doctor if that doesn't help with the snoring. Sleep apnea can have serious health consequences including heart problems.
Consider. Breathe right strips.
Elevate. The first thing to try is sleeping with your head and nose elevated above your heart. The nose is filled with blood vessels and if you sleep flat these vessels will be filled up blocking your nose more. Try irrigating your nose with saline. If this doesn't help it may be due to a cold, allergies or sinus condition and seeing your primary care doctor and if necessary an ENT doctor can help.
Solve your mystery. Why is your nose plugged up at night? Any clues in your bedroom: dust, mildew/mold, bedding, pets? During daytime too? Talk with your doc about your list of prime suspects so you can prevent this. Good luck!