A few thoughts:
1) It could be allergies (my guess here would be dust since it is worse when you go to bed) or acid reflux. Controling either of these with medication or life style changes would help the symptom.
Allergy life style changes include: showering before bed to wash pollen and other allergens out of your hair so that you don't breathe all night as you sleep, sinus rinses, washing bedding in HOT water weekly, allergy proof covers for matress, boxsprings and pillows.
Acid reflux lifestyle changes: no eating 4 hours prior to going to bed, avoid caffeine (coffee, tea, and chocolate), avoid spicy food including onions and garlic, sleep on an incline with your head elevated about 6 inches.
2) As allmymarbles suggested, saline will help, but as a wash/rinse. Drops won't do a whole lot to cleanse the sinuses of allergens, irritants and mucus to reduce the symptoms. Drops are mainly used to ease dryness.
Take care and feel better.
Sounds like a post nasal drip. Yes it would be worse when you lie down. Try dripping a little saline in each nostril. This would help clear the nasal passages.