I have ( had) mine at the top, because it "falls " threw the cage, but I only ran it every so often - mine is the ultra sonic - but the mist was so fine, if I took a deep breath it would make me choke- so I stopped using it until I can find out more about them inhaling the mist - it made me nervous of a URI -
I have mine at the top and it runs all day on dry days. If it starts getting too humid in the room I shut it off. And it always goes off with the lights at night.