I've been stacking them for about a week. 3Z seems to make me sleep longer and harder than HGHpro. HGHpro seems to make five or six hours of sleep seem like seven or eight hours in terms of feeling refreshed.
I've been using a stack of two capsules of each. I actually just took three HGHpro w/two caps of 3Z to see if I feel a difference. I don't think I'll use three caps of 3Z unless I know I can get at least 8 hours of sleep. It seems to make me sleep very hard.
For what it is worth, my wife could tell that I took something the first night that I tried two caps of each. I know because the next morning she wanted to know what I took the night before, lol.