In the heights of the mountains, Peruvian tribesmen get energy by sucking on maca (Lepidium meyenii) plants. This turnip-or radish-shaped vegetable from the mustard family has been used as food and medicine, to promote endurance and improve energy, vitality, sexual virility and even fertility.
The data on its increased energy effects seem strong, but the reported side effect is insomnia. It can be obtained in a powder at many stores (Whole Foods, etc.) or from reputable dealers on the Internet.
A single teaspoon (that's the dose in the studies) can be added to blender drinks, pancakes and other food products. The teaspoon keeps you going all day long. Or take 1/2 gram of the extract in pill form twice a day.