Caffeine - The world's most popular psychoactive drug

National Geographic
"For most of human existence, your pattern of sleeping and wakefulness was basically a matter of the sun and the season," explains Charles Czeisler, a neuroscientist and sleep expert at Harvard Medical School. "When the nature of work changed from a schedule built around the sun to an indoor job timed by a clock, humans had to adapt. The widespread use of caffeinated food and drink—in combination with the invention of electric light—allowed people to cope with a work schedule set by the clock, not by daylight or the natural sleep cycle."