There are many documented causes of hives. The exact cause is never found in a significant number of cases. You should seek relief of your daughter's hives, with an antihistamine prescribed by her doctor who, knowing her and her medical history, may have a clue regarding the causative agent, food or other. To learn more please read our hives (urticaria) information by copying and pasting this address
http://www.nationaljewish.org/disease-info/diseases/allergy/about/conditions/hives.aspx
into the address window of your internet browser. Share this information with your daughter’s doctor to see if she would benefit from any of this.
could it be mold in hte house and that since she is getting better during the day might be because she is at school. I don't have an answer for you sorry, It is just a thought, but try keeping a diary of what she eats wears for clothes, it might be a dye in her jammies at night. anything could be possible. write down when she is worse and when she is bettter maybe you can find the link
good luck
michelle