Caffeine

Caffeine, found in coffee and tea, is the world’s most widely used stimulant; it blocks adenosine receptors to increase alertness and reduce perceived fatigue, indirectly raising dopamine and norepinephrine. Moderate intake is well studied and associated with improved attention and reaction time and, observationally, with lower risk of several chronic diseases. Downsides include tolerance, dependence with withdrawal headaches, anxiety and insomnia at higher doses, and disrupted sleep when taken late.