Caffeine aka Trimethylxanthine

Caffeine is known medically as trimethylxanthine, and the chemical formula is C8H10N4O2. When isolated in pure form, caffeine is a white crystalline powder that tastes very bitter.
Medically, caffeine is useful as a cardiac stimulant and also as a mild diuretic (it increases urine production). Recreationally, it is used to provide a "boost of energy" or a feeling of heightened alertness. It's often used to stay awake longer -- college students and drivers use it to stay awake late into the night. Many people feel as though they "cannot function" in the morning without a cup of coffee to provide caffeine and the boost it gives them.