When caffeine is ingested, it blocks the action of PDE. This chemical normally breaks down a second chemical messenger, known as cAMP. Without being broken down, cAMP causes epinephrine and norepinephrine to be produced in the heart: These chemicals increase heart rate and blood flow.
Epinephrine is the chemical formerly referred to as adrenalin. This neurotransmitter is widely known for providing a sudden burst of energy to an individual who is startled into a fight-or-flight response. During an emergency situation, the increased energy, heat rate and blood flow provides more oxygen to the brain and other tissues. While the amount of epinephrine and norepinephrine released due to caffeine's interference with cAMP breakdown is much smaller, it is sufficient enough to increase an individual's heart rate and blood pressure.
Caffeine has different effects on different people, so the amount of energy and duration of the energy boost provided by caffeine intake depends on each individual. Some consumers drink a cup of coffee right before going to bed and sleep normally, while others would be up all night with the jitters. Individuals who regularly ingest caffeine may experience headaches from caffeine withdrawal, because their previously restricted blood vessels re-dilate in the absence of caffeine.