with prolonged use you can deplete your adrenal glands. adrenaline is what gives you the feeling of alertness and energy. i found this online:
"Caffeine forces your glands to secrete when they don’t have much left to give, and they have to keep digging deeper and deeper, making you more and more tired over time. And over the years, it takes more and more coffee to get the same result. Some people reach the point of drinking half a dozen or more cups of coffee to get the same result and it’s barely keeping them awake. That’s severe adrenal depletion.”
and this
"Within five minutes after you drink your morning coffee, the caffeine begins to stimulate your central nervous system, triggering the release of stress hormones in your body, causing a stress (“fight or flight” ) response. The stress hormones are useful if you need to prepare yourself to fight or flee a dangerous situation, but if you are simply sitting at your desk you may feel a short charge of alertness, quickly followed by feelings of agitation. Within the next hour or so, after the stress response dissipates, you will probably feel more tired and hungry."
off topic but I dont drink caffeine myself and got a little curious so I looked this up, FYI. caffeine gives me wicked jitters even from a little bit so I prefer to just get some sleep and eat well.