«Caffeine is a central nervous system (CNS) stimulant of the methylxanthineclass. It is the world's most widely consumed psychoactive drug. [...] it reversibly blocks the action of adenosine on its receptor and consequently prevents the onset of drowsiness induced by adenosine. Caffeine also stimulates certain portions of the autonomic nervous system.
Caffeine is a bitter, white crystalline purine, a methylxanthinealkaloid, and is chemically related to the adenine and guaninebases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is found in the seeds, nuts, or leaves of a number of plants native to Africa, » (wikipedia)