Actually,
caffeine is a
diuretic and hence pulls the
water out of your system. So, in additon to causing the
adenosine crash, it causes a
hydrological crash as the water in your
body is taken away and you are left without the added
metabolic and
processing benefits of the
water itself -- causing you to
crash, hard.
For more information on the perils of caffeine consumption, direct yourselves to the cautionary tale: Caffeinated Nasal Spray.