The BCAA theory follows along the lines of BCAAs being metabolized by muscle rather than the liver. When doing cardio they, along with glutamine, drop first. A lot of the energy gels used in distance sports have added BCAAs in them to counter this. As BCAA levels drop, in the brain they are overpowered by tryptophan, so you get a rush of serotonin, causing drowsiness. This really happens only in larger cardio events though. Personally, I would use something like that endurox drink with carbs and protein all the time if I could afford it. Probably wouldn't kill me to make my own lol.