the only times i have seen this with the ZMK product was when i did not get a full 8 hours of sleep. with 5 hours, it seemed to give me a very "heavy" feeling. i am sure you can attribute this to still being in a deep stage of sleep. therefore, i will only take 1 or 2 if i know i cannot sleep the full time. also, i have been experimenting with the dosing timing around food intake. it seems to be a little more effective for me when taken with without food but as millennium said above just try to keep calcium out of the mix.
It would also make sense that some people, in general, are not used to having the "flood" of minerals during their sleep period. Full body recuperation is appreciated side effect and i also felt a minor buffering effect.
ZMK is a great product and i have seen much better biofeedback from it than its ZMA predecessors. I love the stuff.
Good Luck DW