Try simple things like more sleep or getting a ZMA supplement.
Using caffeine, ephedrine, vinopocetine, etc will work fine for a temp amount of time but you'll also be more dehydrated, and have lower amt's of DHEA, calcium and/or epinephrine (due to negative efx on adrenal glands). Caffeine efx the levels of many of those things. So I quick fix isn't always the best answer look at your diet, training and sleep to get a better idea of what could be causing your low energy level.
Up your protein for starters and maybe decrease your fat (esp. saturated fat intake) and make wholesome choices for your carbs (ie-complex...oatmeal, veges, fruits, brown rice, etc).
Maybe you're overtraining and that could very well be the cause of your low energy levels. Take a couple days off and buy yourself a mag to build up motivation to get back in the gym and start fresh if needbe.