water

My machine doesn't use any water for the first several hours each night, then uses the entire resevoir during the second half of the night. Why is this?

using the whole resevoir... would guess that you're mouth breathing.

My explanation would be that during the first hour or two, the water is still close to room temperature and doesn't evaporate much. After being warmed by the humidifier's heater, it evaporates more quickly.

If you run the humidifier without the heater on, you will see that not much is gone in the morning.