Caffeine is a chemical that can be found in coffee, tea, cocoa, chocolate, soft drinks, and some medications.  Caffeine belongs to the class of drugs known as stimulants. Stimulants may cause accelerated heart rate, increased blood pressure, and an increase in the rate of the body’s metabolism. Caffeine stimulates the central nervous system by increasing the metabolism inside neurons. Caffeine increases wakefulness by blocking the neurotransmitter, adenosine.

While caffeine can increase alertness, it may also reduce fine motor coordination, alter sleep patterns, and cause headaches, nervousness, and dizziness.

Source: Brain Power! The NIDA Junior Scientist Program