I couldn't find any statements backing up your claim that Melatonin shrinks brains; in fact, I found allegations to the opposite: Melatonin appears to have neuroprotective properties.[0] I suggest you cite your sources, especially when making sweeping claims like "it's well understood as...", "easily found stuff", etc.

[0] http://www.lef.org/magazine/mag2013/sep2013_Melatonin-The-Br...