A sensation of supreme dryness and lack of moisture in the mouth, akin to drinking 8 pints of sand after playing basketball, that wakes the victim up in the middle of the night and creates a feverish, panicked, and borderline predatory need for cold, refreshing liquid. Also known as after hours cotton mouth on steriods.
Jill: So, I have a vague memory of you exploding out of bed, knocking over the TV, and making dinosaur sounds last night. Bad dream?
Frank: No, I woke up with Midnight Mouth and had to chug half a gallon of orange juice.