While sleeping salivation is usually decreased. If you are sleeping and have a blocked nasal tract (rhinitis, allergies) or just an open mouth you further dry out your mouth. Other causes might also come into play. As stated in the linked article above, this is not only uncomfortable in the morning, it is detrimental to the health of your teeth.
What safe possibilities might exist to increase saliva production during non-waking hours? (Assuming that pregnancies aren't always 'that' practical…)