I've been taking 400mg of Magnesium Glycinate two times a day along with 200mg of Theanine (also twice a day) with my usual two cups of coffee. I feel fantastically focused without the jittery edge that accompanies caffeine alone.
I thought it was all garbage but I tried it and noticed I slept much deeper than normal and through the entire night, consistently. I have no idea if this is all placebo effect, but it's interesting hearing others have similar effects.