Coffee contains caffeine, which is a type of stimulant that prevents the onset of drowsiness and increases the energy levels of an individual. Caffeine is considered as a psychoactive drug that belongs to the xanthene class of stimulants. Unlike most psychoactive drugs, however, caffeine is unregulated and is classified by the United States Food and Drug Administration as "generally regarded as safe."
The onset of sleepiness is caused by the gradual slowing down of nerve cell activity in the brain, which in turn is caused by a neurotransmitter in the brain called adenosine when it binds to adenosine receptors in the brain. The act of binding activates the adenosine receptors, which triggers a cellular response that increases sleepiness.
Caffeine mimics adenosine and it can also bind with the adenosine receptors in the brain. However, instead of increasing drowsiness, the binding of caffeine with the adenosine receptors antagonizes the receptors. This means that caffeine binding with the adenosine receptors causes an different cellular response by increasing wakefulness and at the same time block the real adenosine from binding with adenosine receptors.
For most people, the effective dose of caffeine to increase alertness and wakefulness is about 100 to 200 milligrams, notes WebMD. A 5-ounce cup of coffee contains about 100 milligrams of caffeine, but the actual caffeine content may vary depending on the strength of the coffee used.