Pains and illnesses related to the part of the body where it crosses. Even the top bunk might be preferable to the lower bunk just on that basis. If it is absolutely not possible, a hanging cloth draped loosely around the lower part of the bunk to smooth the edges (if there is enough space, let it hang like a curtain to give the illusion that there is nothing behind the cloth but if not then do what you can) will help.
