"Caffeine also can help open the box that tells your body to go into extra energy mode. Things like your heart can work faster or slower depending on what you need. If you are sitting on the couch watching TV it's going to go slower, if you are outside working it's going to speed up. Caffeine tricks the body into thinking it needs to go into extra energy mode. Caffine doesn't create this energy, the body is just using what it has stored more quickly. Not really any different from you step on the gas in a car. You are telling it to burn more fuel and go faster."