There will be absorbtion issues persay if your using something like zinc glycinage or a lowbioavilibity form of zinc, especially with a casein shake, something like zinc citrate or picolinate will greatly enhance the effects of zinc in your bloodstream. I take zinc first thing in the morning and on an empty stomach, I'm really pleased with the Zinc in picolonate form and have had no stomach issues whatsoever, I was using the L-optizinc ( which is methionine bound ) and that tore my stomach up.
There is no benefit IMO taking zinc @night versus taking it in the day, as long as your getting it is what's important.