A large burst of caffeine does cause an adrenaline surge. As a matter of fact, that's all that does anything to wake people up who drink it in the early morning

A large burst of caffeine does cause an adrenaline surge. As a matter of fact, that's all that does anything to wake people up who drink it in the early morning. However caffeine is a competitive inhibitor of adenosine, a neurochemical that builds up late in the day making us sleepy. So if you drink it in the afternoon, especially in smaller quantities, as you'd see in an espresso (Which while having a stronger coffee taste, actually has much less caffeine per serving) or tea, you will find it quite able to keep you awake.