There are lots of reasons you could be having a dry mouth at night.
Some medications make the mouth dry too. If you're taking a particular medication in the late afternoon or early night, it might be the culprit. Some types of illnesses cause dry mouth syndrome. If it's happening only at night you could try using a humidifier or perhaps you are dehydrated.

You could search for Dry Mouth or Dry Mouth Syndrome in the forum search feature
http://neurotalk.psychcentral.com/search.php