Im just taking a wild stab in the dark here. Perhaps its the way that it mimicks some of the effects of thyroxine. I now that its been shown to lower the levels of thyroid hormone, and this shows that it act in a similar fashion and has a negative feedback loop. It stimulates the metabolism, similar to taking something ephederine. This would keep you up at night. I know it did me. I also know that it is quite effective at jacking up your blood pressure too. This shows how it has sympathetic activity similar to norepineherine (aka adrenaline). Norephinepherine is part of that "fight or flight" response.