Re: Dry mouth at night. Poor sleep.
I'm not sure, but I have been having almost the same thing happen during this fast (also on day 17!). It started about 3 or 4 days ago and I've been having dry mouth both while I sleep (the very little that I do) and during the day. I don't feel excessively thirsty, but have been drinking significantly more water to try to hydrate my mouth!
Everything else feels fine, but I do miss sleeping at normal hours! I can definitely sympathize with you!!
I'm not sure the cause, but I tend to agree with what everyone else says: you might just have to ride it out! I'd guess, depending on what the weather is like where you live, that could be a factor, as well; I tend to get much more "dried out" (sinuses and mouth) during the winter due to the drier air and use of heaters. Have you ever tried a room humidifier? Perhaps that could help? It certainly can't hurt!
Good luck with the rest of your fast...how many days are you going for? I will be doing 24. :)