I think I may be able to give you some advice nohorn but first of all, if you'll bear with me, I'd like to check if your lifestyle may be contributing to the problem.
First of all, do you suffer from high blood pressure? If so, are you taking diuretics? These can contribute towards a dry mouth in a big way and if you are taking them, it may be worth having a chat with your GP to see if a different type may suit you better.
The next thing I'd suggest you do is to avoid tea and coffee as both of these can cause a type of diuresis that shows itself in the form of a dry mouth. Try cutting them out in the late afternoon and/or evening to see if it makes a difference.
Are you taking any anticholinergic drugs? These are renowned to cause the symptoms you describe. A typical example is ipratropium bromide used to treat COPD and similar conditions. Opioid analgesics and cardiac drugs such as Clonidine can also do it. Also avoid any mouthwashes containing alcohol.
If this xerostomia occurred in the daytime as well, I'd suggest you need to avoid mouth breathing at all costs. I do appreciate that this is impossible while your asleep. All the same, the salivary glands do atrophy as we get older and it's not unusual for people to have to drink water during the night, so what you're describing is not that unusual.
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