Energy drinks and soda, both have caffeine, which is technically a drug. And for good reason too! You can become physically addicted to caffeine, which is why many people buy soda before they buy milk or orange juice.
Like any drug, you can also build up a tolerance for caffeine. This explains why so many people can fall asleep after drinking a can of pepsi. Energy drinks, however, work a bit differently. Energy drinks are filled with huge amounts of caffeine. If you drink an energy drink every day, you will start to feel down and you will have a lack of energy when you stop drinking energy drinks. Just ask anyone who stopped drinking coffee for the first time in weeks.