I believe the term for this is "insensible respiration". My weight doesn't fluctuate as much as yours does overnight, but it can fluctuate several pounds. My waist can fluctuate several inches between night and morning, too. I, too, can sleep with minimal clothing and no covers (have had to learn to sleep without covers since I sleep with a cover hog).
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 180 x 2. Current Deadlift: 230 x 2