When we talk to other people about achieving goals, we often speak in terms that relate to energy. Psychologists talk about the energy that is related to achieving goals as arousal.
The researchers analyzed the caffeine content of 10 of the best-selling energy drinks along with 19 types of carbonated soda and seven other best-selling commercial beverages. PepsiCo's Mountain Dew had the most caffeine of any regular soft drink tested, with 45 milligrams per 12-ounce serving.
For most people, occasional energy drinks are fine, but try to limit yourself to about 16 ounces (500 milliliters) a day. If you're consistently fatigued or rundown, however, consider a better and healthier way to boost your energy.