Causes for increased salivation during sleep could be many most probably stress and autonomic nervous system imbalance since salivation is controlled by autonomic nervous system to a great extent so subconscious mind if stress exist there can be increased salivation during sleep. So try to be calm at bedtime. Drink more water during bedtime as hydration will help to adjust salivation to normal level during sleep. Oral hygiene is another important factor which can affect salivation. So clean your mouth and brush your teeth regularly at bedtime. If symptoms don't get relief consult ENT specialist.