Apart from treatment of the cause or
in conditions wherein the cause remains unknown,
in such conditions Drinking lot of water and taking proper oral hygiene will help you. You can take citrus juices, lemonade and foods marinated in vinegar, salt water gargling, proper brushing etc.
You can use scented wax or incense sticks before sleeping. You also need to maintain good sleep hygiene.
I suggest you to consult ENT specialist to rule out the cause. Take care and regards.