I don't know why but ZMA fucks up my sleep too. I become a VERY light sleeper when I'm on it. Even if I'm not woken by a pin drop, I will wake up automatically at 4AM or so then find it hard to get back to sleep. I can't sleep more than 6-7 hours on ZMA, it's just impossible. It's a shame too because like Vedic said, a lot of athletes are magnesium deficient (even though I'm not an athlete).
I've never been a good sleeper, but yea very light definitely applies - a lot of the times i will 'think' whilst im in the dream, and my sleep is so light ill literally drift back into awakeness, thinking i was trying to do something in the dream. f**k that.
But yea, I take a couple in the morning if I can, because I'm sure I feel better when Im on the zma, but hey could be a placebo as well. it's weird.