Why is my hamster sleeping under her water bottle and not her hide?

Hello there! It's not entirely surprising that Arwen has chosen her water bottle to hide under - this is fairly common for hamsters. As much as we want them to use all the awesome hiding structures we purchase for them, they always tend to pick something else, to our dismay (or turn fancy huts into boats) :) I don't think that there is any medical concern for Arwen unless she is acting strange, or isn't eating and drinking normally. If anything, a rodent that chooses to sleep in an unguarded location as opposed to in a hut is generally assumed to be comfortable with its surroundings, so that is a positive thing. My only concern with being under the water source is if there is any moisture or dripping from the bottle where she is sleeping - this could cause skin issues. You might consider moving the bottle to another location. Of course, if she chooses to move her bed underneath the bottle again, you'll just have to keep a close eye out for excess moisture in that location. Good luck, and feel free to post again if you have any other concerns!