During normal circadian rhythms, an increase in adenosine will trigger regulatory pathways to induce sleep. The binding of adenosine causes drowsiness by slowing down nerve cell activity. In the brain, adenosine binding also causes blood vessels to dialate. Caffeine is a substrate inhibitor for adenosine; binding to its receptors and effectively cancelling out the sleepy effects of adenosine. Instead of slowing down because of the adenosine level, the cells speed up. Caffeine also causes the brain's blood vessels to constrict, because it blocks adenosine's ability to open them up.
The crash comes when the body begins to degrade the caffeine. Via normal equilibrium determinants in the body, adenosine will be in a higher concentration in its unbound form hence being readily accessable to the receptors once caffeine is released. Thus unless more caffeine is quickly ingested, your body will feel a massive urge to sleep.
The moral of the story for everyone,but students in particular, is to avoid caffeine when staying up all night because the next day during your exam you will CRASH. Remember that the half-life of caffeine is 6 hours, its effects can be long-lasting but when it gets degraded, adenosine will quickly combat caffeine's artificial effects.

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