Dr. James Bibb, associate professor at University of Texas Southwestern Medical Center, says caffeine is the oldest stimulant known to man.
"Caffeine has proven to be a useful drug," Bibb says in a statement. "But, as with all things, moderation is required so that we receive its benefits."
Caffeine stimulates people in two ways -- it ultimately facilitates the release of acetylcholine, a chemical compound that prevents drowsiness and also blocks a type of adenosine receptor located in the part of the brain that controls reward-based learning.
By blocking those receptors and keeping the brain primed for rewards, caffeine keeps people alert, Bibb says.
However, caffeine can also raise the heart rate, cause motor or muscle tremors, increase blood pressure and act as a diuretic that reduces water content in the blood -- which might also affect sensory neurons in the dura, the thin envelop of tissue that surrounds the brain and may be the source of headaches.
"A small amount of caffeine can be very helpful, allowing us to work with focus and attention and stay aroused and awake," Bibb says. "But too much of it can be unhealthy."