Having a dry mouth at night or first thing in the morning is nearly always a sign of excess mouth breathing - which in turn nearly always points to a blockage to the free flow of air through the nose.
Many people with a dry mouth in the morning also snore at night. The air simply cant get through the blockage in the nose (or at the back of the nose) and this forces the person to breath through the mouth. The mouth has to stay open as they sleep and this flow of air causes the normal saliva to dry by evaporation.
Some medications also trigger a dry mouth and this too is something for the doctor to look at.
In some cases the nasal blockage can be improved by using steroid nose sprays or steroid nose drops. Sometimes an ENT surgeon needs to operate to resolve things.
Sleeping in a very warm room can aggravate things. Warmer air dries more saliva as the person sleeps with an open mouth, so one simple thing to do is to turn the heating down a bit or open the window. Making the air more moist by using water near heaters can also help.