Dry mouth is one of the symptoms that can arise if acid flows upwards from the stomach towards the mouth. This is known as acid reflux.
The stomach lining is designed to cope with acid - the lining of the gullet and throat is not.
If you are creating excess acid in the stomach - or if the muscular valve at the top of the stomach is not working properly then acid will leak back upwards and cause irritation as it goes. This often leads to a dry mouth - or to a bad taste in the mouth and bad breath. The problem is always worse when lying flat or when stooping forwards so night time is often a problem.
If you think this applies to you then you should see your doctor for advice. You might be sent for tests to confirm the diagnosis - or you may simply be given a trial of treatment to see if it helps in the first instance.