Discovery Health "Caffeine and Adenosine"

"The problem with caffeine is the longer-term effects, which tend to
spiral. For example, once the adrenaline wears off, you face fatigue and
depression. So what are you going to do? You consume more caffeine to
get the adrenaline going again. As you might imagine, having your body
in a state of emergency all day long isn't very healthy, and it also
makes you jumpy and irritable.

The most important long-term problem is the effect that caffeine has on
sleep. Adenosine reception is important to sleep, and especially to deep
sleep. The half-life of caffeine in your body is about six hours. That
means that if you consume a big cup of coffee with 200 mg of caffeine in
it at 3:00 p.m., by 9:00 p.m. about 100 mg of that caffeine is still in
your system. You may be able to fall asleep, but your body will probably
miss out on the benefits of deep sleep."

http://health.howstuffworks.com/wellness/drugs-alcohol/caffeine3.htm