The idea is based on the amount of moisture in the air and the transference into or out of the lungs. In many dehydration cases in humans, sweating is not always the main culprit in water loss. Many will lose water through breathing in dry rooms at night sleeping. The effect is amplified for those who breathe through their mouths while sleeping. It is finding a balance. If you have two rooms separated by a door and one has 20% humidity and the other 1000%, once the door is left open the two rooms will start to become balanced. The molecules in the dry air have more room between them and it is easy for moisture to take up the space.
Think of a cold winter day. You go outside and see your breath. That is moisture leaving your body through your lungs. (There is also a temperature factor here as well.) That moisture transfer happens in the chameleon's natural environment and we can replicate it in our captive settings with a fogger. If we only focus on misting during the day for hydration we could actually be causing more dehydration due to our methods. I will say that tons of us have been successful using misting as the primary source of water, but this seemed like a more efficient way to provide it. That is what led me to try the night time fogging method and I am also very pleased with the results.
I'm sure someone here has the terms that explain the moisture transfer in lungs as well as how it aids in oxygen use by the body, but that is above my pay grade

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