it usually has a relaxing effect so take it before bed, or maybe first thing in the morning

That wouldn't screw up the backload if I take before bed?I forgot to mention this in the OP, but I use Biotest's ElitePro, which also has chromium and VS, which have some kind of GDA-like effect and I'm wondering if I should be taking those at all because I've read the Kiefer dicourages GDAs during the backload as it defeats the point. But I'm really just concerned about the magnesium as I need that to prevent insomnia.

Just take it man! Its not going to have that big effect on your insulin sensitivity that itll ruin your backloading effects. Good sleep is far more important. I take Magnesium every night before bed and I dont think its hurt my progress on CBL at all.

I'm going to take it no matter what, because you're right, sleep is way more important. I was just wondering if anyone had a suggestion as to something to minimize the negative effects it may have on the backload. I think I'll just try and take it after I'm done eating.

the other thing you could do is split the dosage through out the day before your backload. You wont get the same sleepiness at bedtime but the effect on your ability to relax (and thus get to sleep easier) should still remain.